Add symbol visibility information to all the functions that are exported

[imciner2]

This adds the symbol visibility information to the main API functions that QDLDL exports so that they are exposed inside shared libraries on all platforms. This is mainly an issue on Windows (which defaults to exporting no symbols in a shared library), but could also be a problem if someone tries to use the flag -fvisibility=hidden on other platforms.

Before this change (compiled with -fvisibility=hidden), readelf -Ws out/libqdldl.so shows:

Symbol table '.dynsym' contains 5 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterTMCloneTable
     2: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
     3: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_registerTMCloneTable
     4: 0000000000000000     0 FUNC    WEAK   DEFAULT  UND __cxa_finalize@GLIBC_2.2.5 (2)

After this change (compiled with -fvisibility=hidden still), readelf -Ws out/libqdldl.so shows:

Symbol table '.dynsym' contains 10 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterTMCloneTable
     2: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
     3: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_registerTMCloneTable
     4: 0000000000000000     0 FUNC    WEAK   DEFAULT  UND __cxa_finalize@GLIBC_2.2.5 (2)
     5: 0000000000001240  1120 FUNC    GLOBAL DEFAULT   11 QDLDL_factor
     6: 00000000000016a0    96 FUNC    GLOBAL DEFAULT   11 QDLDL_Lsolve
     7: 0000000000001700    89 FUNC    GLOBAL DEFAULT   11 QDLDL_Ltsolve
     8: 0000000000001760   211 FUNC    GLOBAL DEFAULT   11 QDLDL_solve
     9: 0000000000001120   278 FUNC    GLOBAL DEFAULT   11 QDLDL_etree

I am marking as a draft since I still want to do some testing on platforms other than Linux.

[coveralls]

Pull Request Test Coverage Report for Build 176

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 170:	0.0%
Covered Lines:	87
Relevant Lines:	87

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1221859168

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 1065854028:	0.0%
Covered Lines:	87
Relevant Lines:	87

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💛 - Coveralls