Introduce prototype-style function definition for generic stpcpy.
Upstream part of:

commit 142e0a9953059bcf5667921f2670efec377be3d5
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Tue Jul 28 17:12:25 2015 -0300

    powerpc: Use default strcpy optimization for POWER7

diff --git a/string/stpcpy.c b/string/stpcpy.c
index b74f6e2b489e1f18..8b2c4c5bb7e1ee74 100644
--- a/string/stpcpy.c
+++ b/string/stpcpy.c
@@ -36,9 +36,7 @@ extern __typeof (__stpcpy) STPCPY;
 
 /* Copy SRC to DEST, returning the address of the terminating '\0' in DEST.  */
 char *
-__stpcpy (dest, src)
-     char *dest;
-     const char *src;
+__stpcpy (char *dest, const char *src)
 {
   char *d = dest;
   const char *s = src;