Always include <sys/cdefs.h>.

I made a mistake caught by code review earlier, so let's try to be
safer by default.

This patch also moves all our "forwarding" headers to the guardless
just-include-the-other-thing style that we usually use. (Where we
have a comment explaining where the header comes from, I've kept
that.)

Change-Id: I37342cf5e2563c6a269b2ba61a697069b1c7913b
diff --git a/libc/include/xlocale.h b/libc/include/xlocale.h
index f7eb8f4..559d24d 100644
--- a/libc/include/xlocale.h
+++ b/libc/include/xlocale.h
@@ -29,6 +29,8 @@
 #ifndef _XLOCALE_H_
 #define _XLOCALE_H_
 
+#include <sys/cdefs.h>
+
 /* If we just use void* here, GCC exposes that in error messages. */
 struct __locale_t;
 typedef struct __locale_t* locale_t;