Cellular: Include of errno.h needed for Mac OS X

Author: Mirela Chirica

features/cellular/framework/AT/ATHandler.cpp

@@ -18,6 +18,7 @@
 #include <ctype.h>
 #include <stdio.h>
 #include <limits.h>
+#include <errno.h>
 #include "ATHandler.h"
 #include "mbed_poll.h"
 #include "FileHandle.h"