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/android/api-level.h b/libc/include/android/api-level.h
index b230cef..001877b 100644
--- a/libc/include/android/api-level.h
+++ b/libc/include/android/api-level.h
@@ -29,6 +29,8 @@
 #ifndef ANDROID_API_LEVEL_H
 #define ANDROID_API_LEVEL_H
 
+#include <sys/cdefs.h>
+
 /*
  * Magic version number for a current development build, which has
  * not yet turned into an official release.