Added const modifier to String.c_str()

Author: cmaglie

hardware/arduino/cores/arduino/WString.h

@@ -147,7 +147,7 @@ class String
 	void getBytes(unsigned char *buf, unsigned int bufsize, unsigned int index=0) const;
 	void toCharArray(char *buf, unsigned int bufsize, unsigned int index=0) const
 		{getBytes((unsigned char *)buf, bufsize, index);}
-	char* c_str() const { return buffer; }
+	const char * c_str() const { return buffer; }
 
 	// search
 	int indexOf( char ch ) const;