string: Add libc_hidden_proto for memrchr

Although static linker can optimize it to local call, it follows the
internal scheme to provide hidden proto and definitions.

Reviewed-by: Carlos Eduardo Seo <carlos.seo@linaro.org>

string/memrchr.c

@@ -75,5 +75,6 @@ __memrchr (const void *s, int c_in, size_t n)
   return NULL;
 }
 #ifndef MEMRCHR
+libc_hidden_def (__memrchr)
 weak_alias (__memrchr, memrchr)
 #endif