Fixed another regression in IPAddress.h

If the includer tries to inlcude IPAddress.h without first including
WString.h the build will fail.

Author: cmaglie

avr/cores/arduino/IPAddress.h

@@ -21,7 +21,8 @@
 #define IPAddress_h
 
 #include <stdint.h>
-#include <Printable.h>
+#include "Printable.h"
+#include "WString.h"
 
 // A class to make it easier to handle and pass around IP addresses