deps/glad: Fix build with GCC-10

[Chiitoo]

Description

Use 'extern' to turn the first definition of 'gladGetProcAddressPtr' into a declaration.

Motivation and Context

GCC-10 defaults to '-fno-common', which triggers issues with multiple definitions of global variables.

• https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85678

Without the changes, builds will fail like so:

FAILED: deps/glad/libobsglad.so.0 
: && /usr/bin/x86_64-pc-linux-gnu-gcc -fPIC -Wall -Wextra -Wvla -Wno-unused-function -Werror-implicit-function-declaration -Wno-missing-braces -Wno-missing-field-initializers -march=znver1 -O2 -fomit-frame-pointer -pipe -mindirect-branch=thunk -std=gnu99 -fno-strict-aliasing  -Wl,-O1 -Wl,--as-needed -shared -Wl,-soname,libobsglad.so.0 -o deps/glad/libobsglad.so.0 deps/glad/CMakeFiles/glad.dir/src/glad.c.o deps/glad/CMakeFiles/glad.dir/src/glad_glx.c.o  /usr/lib64/libX11.so  -ldl  /usr/lib64/libGL.so && :
/usr/lib/gcc/x86_64-pc-linux-gnu/10.0.1/../../../../x86_64-pc-linux-gnu/bin/ld: deps/glad/CMakeFiles/glad.dir/src/glad_glx.c.o:(.bss+0x4b8): multiple definition of `gladGetProcAddressPtr'; deps/glad/CMakeFiles/glad.dir/src/glad.c.o:(.bss+0x5008): first defined here
collect2: error: ld returned 1 exit status

How Has This Been Tested?

Build tests ran with GCC 10.0.1 and 9.3.0 on Gentoo Linux.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.