issue #4660: Warning fix in CCLabel.cpp and ConvertUTF.c

cocos/2d/CCLabel.cpp

@@ -1193,7 +1193,7 @@ void Label::computeStringNumLines()
 
 int Label::getStringLength() const
 {
-    return _currentUTF16String.length();
+    return static_cast<int>(_currentUTF16String.length());
 }
 
 // RGBA protocol

external/ConvertUTF/ConvertUTF.c

@@ -52,6 +52,8 @@
 #include <stdio.h>
 #endif
 
+#include <string.h>
+
 static const int halfShift  = 10; /* used for shifting by 10 bits */
 
 static const UTF32 halfBase = 0x0010000UL;