string: Add libc_hidden_proto for strchrnul

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/strchrnul.c

@@ -51,5 +51,6 @@ __strchrnul (const char *str, int c_in)
   return (char *) word_ptr + index_first_zero_eq (word, repeated_c);
 }
 #ifndef STRCHRNUL
+libc_hidden_def (__strchrnul)
 weak_alias (__strchrnul, strchrnul)
 #endif