Reduce the exposure of the __set_errno implementation detail.

Change-Id: I395e1b46a9491e34fc53e71853e932ea90b3d1cc

libc/include/errno.h

@@ -40,10 +40,6 @@ __BEGIN_DECLS
 #define  ENOTSUP  EOPNOTSUPP
 #endif
 
-/* internal function that should *only* be called from system calls */
-/* use errno = xxxx instead in C code                               */
-extern int    __set_errno(int  error);
-
 /* internal function returning the address of the thread-specific errno */
 extern volatile int*   __errno(void);