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/sys/cachectl.h b/libc/include/sys/cachectl.h
index 0830293..5b20c19 100644
--- a/libc/include/sys/cachectl.h
+++ b/libc/include/sys/cachectl.h
@@ -25,10 +25,13 @@
  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
+
 #ifndef _SYS_CACHECTL_H
 #define _SYS_CACHECTL_H 1
 
+#include <sys/cdefs.h>
 #ifdef __mips__
 #include <asm/cachectl.h>
 #endif
+
 #endif /* sys/cachectl.h */