CPP: Fix compilation error on Clang.

This fixes a compilation error on Clang due to conflicting declarations (struct
vs class) in multiple header files.

BUG=http://crbug.com/236272
R=jia.shao.peng@gmail.com

Review URL: https://codereview.appspot.com/9259043

cpp/src/phonenumbers/callback.h

@@ -26,7 +26,8 @@ namespace i18n {
 namespace phonenumbers {
 
 template <typename R, typename A1, typename A2, typename A3, typename A4>
-struct ResultCallback4 {
+class ResultCallback4 {
+ public:
   virtual ~ResultCallback4() {}
   virtual R Run(A1 a1, A2 a2, A3 a3, A4 a4) = 0;
 };