libc: tag missing functions in system headers.

This matches recent changes in the NDK header.
We enclose missing functions in #if 0 .. #endif blocks
with a clear "MISSING" in comments in order to locate
them later.

Change-Id: I87b3a62e777897e75c9243360fb0a82bcc53d9fb

libc/include/stdio.h

@@ -300,8 +300,10 @@ __END_DECLS
 #define L_cuserid	9	/* size for cuserid(); UT_NAMESIZE + 1 */
 
 __BEGIN_DECLS
+#if 0 /* MISSING FROM BIONIC */
 char	*ctermid(char *);
 char	*cuserid(char *);
+#endif /* MISSING */
 FILE	*fdopen(int, const char *);
 int	 fileno(FILE *);