libc: Add <android/api-level.h>

Add a new header that defines __ANDROID_API__ as a constant integer
corresponding to the current native API level. This header is included
by <sys/cdefs.h>

Change-Id: Ib4774e247b395991266245815a24292afc919848
NOTE: This header is already provided by the NDK's platform headers.
      for levels 3 to 9.

libc/include/sys/cdefs.h

@@ -497,5 +497,6 @@
 #endif
 
 #define  __BIONIC__   1
+#include <android/api-level.h>
 
 #endif /* !_SYS_CDEFS_H_ */