Added wifi fxns as shells where unsupported

Allows more flexibility downstream

src/TinyGsmClientSIM800.h

@@ -462,6 +462,12 @@ public:
     return false;
   }
 
+  /*
+   * WiFi functions
+   */
+  bool networkConnect(const char* ssid, const char* pwd) TINY_GSM_ATTR_NOT_AVAILABLE;
+  bool networkDisconnect() TINY_GSM_ATTR_NOT_AVAILABLE;
+
   /*
    * GPRS functions
    */