Replace is_enum with is_proto_enum because is_enum is not supported on some platforms.

src/google/protobuf/repeated_field_reflection.h

@@ -38,6 +38,8 @@
 #include <google/protobuf/stubs/shared_ptr.h>
 #endif
 
+#include <google/protobuf/generated_enum_reflection.h>
+
 namespace google {
 namespace protobuf {
 namespace internal {
@@ -273,7 +275,7 @@ struct RefTypeTraits<
 
 template<typename T>
 struct RefTypeTraits<
-    T, typename internal::enable_if<internal::is_enum<T>::value>::type> {
+    T, typename internal::enable_if<is_proto_enum<T>::value>::type> {
   typedef RepeatedFieldRefIterator<T> iterator;
   typedef RepeatedFieldAccessor AccessorType;
   // We use int32 for repeated enums in RepeatedFieldAccessor.