Introduce protype-style function definition for S390 pthread_once.  The
old-style function definition was removed upstream in:

commit bc89c0fc70ba952f78fc27fc261ec209be0a6732
Author: Torvald Riegel <triegel@redhat.com>
Date:   Mon Dec 8 18:32:14 2014 +0100

    Remove custom pthread_once implementation on s390.

diff --git a/nptl/sysdeps/unix/sysv/linux/s390/pthread_once.c b/nptl/sysdeps/unix/sysv/linux/s390/pthread_once.c
index cd53d77c163661c5..4bce7fec13ea3bb2 100644
--- a/nptl/sysdeps/unix/sysv/linux/s390/pthread_once.c
+++ b/nptl/sysdeps/unix/sysv/linux/s390/pthread_once.c
@@ -34,9 +34,7 @@ clear_once_control (void *arg)
 
 
 int
-__pthread_once (once_control, init_routine)
-     pthread_once_t *once_control;
-     void (*init_routine) (void);
+__pthread_once (pthread_once_t *once_control, void (*init_routine) (void))
 {
   while (1)
     {