Deprecate readdir_r.

I've been campaigning against readdir_r for years:

  http://elliotth.blogspot.com/2012/10/how-not-to-use-readdirr3.html

and glibc deprecated it in 2.24, so we should join the club.

Bug: http://b/32678321
Test: builds
Change-Id: Ie3887ed75989aced9324266aa3b93e85c11bdbc9

libc/include/dirent.h

@@ -80,8 +80,8 @@ DIR* opendir(const char* __path);
 DIR* fdopendir(int __dir_fd);
 struct dirent* readdir(DIR* __dir);
 struct dirent64* readdir64(DIR* __dir) __INTRODUCED_IN(21);
-int readdir_r(DIR* __dir, struct dirent* __entry, struct dirent** __buffer);
-int readdir64_r(DIR* __dir, struct dirent64* __entry, struct dirent64** __buffer) __INTRODUCED_IN(21);
+int readdir_r(DIR* __dir, struct dirent* __entry, struct dirent** __buffer) __attribute__((__deprecated__("readdir_r is deprecated; use readdir instead")));
+int readdir64_r(DIR* __dir, struct dirent64* __entry, struct dirent64** __buffer) __INTRODUCED_IN(21) __attribute__((__deprecated__("readdir64_r is deprecated; use readdir64 instead")));
 int closedir(DIR* __dir);
 void rewinddir(DIR* __dir);
 void seekdir(DIR* __dir, long __location) __INTRODUCED_IN(23);