os: Squash missing declaration warning for timingsafe_memcmp

timingsafe_memcmp.c:21:1: warning: no previous prototype for ‘timingsafe_memcmp’ [-Wmissing-prototypes]
 timingsafe_memcmp(const void *b1, const void *b2, size_t len)

Signed-off-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 5c44169cae)

os/timingsafe_memcmp.c

@@ -16,6 +16,9 @@
 
 #include <limits.h>
 #include <string.h>
+#include <X11/Xfuncproto.h>
+#include <dix-config.h>
+#include "os.h"
 
 int
 timingsafe_memcmp(const void *b1, const void *b2, size_t len)