ZephyrUDP: add missing pragma once into header file

This is needed to avoid multiple definitions error

Author: pennam

libraries/SocketWrapper/ZephyrUDP.h

@@ -17,6 +17,8 @@
   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
 
+#pragma once
+
 #include "Arduino.h"
 #include "SocketWrapper.h"
 #include "api/Udp.h"