Move the meat of <features.h> into <sys/cdefs.h>.

This way it's a lot harder for us to screw up (since we should always
be including <sys/cdefs.h> anyway).

Bug: 14659579
Change-Id: I23070fff3296b0d1c683bb5e3a6e214146327d53

libc/include/unistd.h

@@ -25,10 +25,10 @@
  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
+
 #ifndef _UNISTD_H_
 #define _UNISTD_H_
 
-#include <features.h>
 #include <stddef.h>
 #include <sys/cdefs.h>
 #include <sys/types.h>