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/alloca.h b/libc/include/alloca.h
index 0c50fc3..0508d31 100644
--- a/libc/include/alloca.h
+++ b/libc/include/alloca.h
@@ -25,9 +25,12 @@
  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
+
 #ifndef _ALLOCA_H
 #define _ALLOCA_H
 
+#include <sys/cdefs.h>
+
 #define alloca(size)   __builtin_alloca(size)
 
 #endif /* _ALLOCA_H */