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/bits/posix_limits.h b/libc/include/bits/posix_limits.h
index 31016a8..0f17381 100644
--- a/libc/include/bits/posix_limits.h
+++ b/libc/include/bits/posix_limits.h
@@ -29,6 +29,7 @@
 #ifndef _BITS_POSIX_LIMITS_H_
 #define _BITS_POSIX_LIMITS_H_
 
+#include <sys/cdefs.h>
 
 /* Any constant values here other than -1 or 200809L are explicitly specified by POSIX.1-2008. */
 /* Keep it sorted. */