add prototypes for GNU *_unlocked stdio functions

actually these are just weak aliases for the normal locking versions
right now, and they will probably stay that way since making them
lock-free without slowing down the normal versions would require
significant code duplication for no benefit.

include/stdio.h

@@ -161,6 +161,10 @@ int vasprintf(char **, const char *, va_list);
 void setlinebuf(FILE *);
 void setbuffer(FILE *, char *, size_t);
 int fpurge(FILE *);
+int fgetc_unlocked(FILE *);
+int fputc_unlocked(int, FILE *);
+char *fgets_unlocked(char *, int, FILE *);
+int fputs_unlocked(const char *, FILE *);
 #endif
 
 #ifdef __cplusplus