fix dependency of String on char lib

Author: ianfixes

cpp/arduino/WCharacter.h

@@ -1,3 +1,5 @@
+#pragma once
+
 inline bool isAlpha(int c) {
   unsigned int data[16] = {
     0x0000, 0x0000, 0x0000, 0x0000,

cpp/arduino/WString.h

@@ -5,6 +5,7 @@
 #include <algorithm>
 #include <iostream>
 #include "AvrMath.h"
+#include "WCharacter.h"
 
 typedef std::string string;