Avoid forward declaration of class EMAC

This hits the name collision with the Nuvoton header files.
Include "EMAC.h" instead to ensure we pick up the #undef workaround.

Author: kjbracey

features/netsocket/OnboardNetworkStack.h

@@ -20,8 +20,7 @@
 #include "nsapi.h"
 
 #include "NetworkStack.h"
-
-class EMAC;
+#include "EMAC.h"
 
 /**
  * mbed OS API for onboard IP stack abstraction