Introduce prototype-style function definitin for generic memchr.

Upstream did this as part of:

commit 1570a72bb8359c9a3a64c4e22d9ea47ce6fcff31

Author: Will Newton <will.newton@linaro.org>

Date:   Thu Jun 26 16:00:44 2014 +0100

    string/memchr.c: Merge from gnulib

diff --git a/string/memchr.c b/string/memchr.c

index d1880cfbaed951a6..f4650672e01385de 100644

--- a/string/memchr.c

+++ b/string/memchr.c

@@ -57,10 +57,7 @@

 /* Search no more than N bytes of S for C.  */

 __ptr_t

-MEMCHR (s, c_in, n)

-     const __ptr_t s;

-     int c_in;

-     size_t n;

+MEMCHR (const __ptr_t s, int c_in, size_t n)

 {

   const unsigned char *char_ptr;

   const unsigned long int *longword_ptr;