c++ remove conversion warning in MapEntryFuncs::ByteSizeLong

[chrisse74]

Removes following conversion warning:

google/protobuf/map_entry_lite.h(131): warning C4267: 'argument': conversion from 'size_t' to 'google::protobuf::uint32', possible loss of data
google/protobuf/map_entry_lite.h(127): note: while compiling class template member function 'size_t google::protobuf::internal::MapEntryFuncs<Key,Value,google::protobuf::internal::WireFormatLite::TYPE_STRING,google::protobuf::internal::WireFormatLite::TYPE_BYTES>::ByteSizeLong(const Key &,const Value &)'
        with
       [
            Key=std::string,
            Value=std::string
        ]
note: see reference to function template instantiation 'size_t google::protobuf::internal::MapEntryFuncs<Key,Value,google::protobuf::internal::WireFormatLite::TYPE_STRING,google::protobuf::internal::WireFormatLite::TYPE_BYTES>::ByteSizeLong(const Key &,const Value &)' being compiled
        with
        [
            Key=std::string,
            Value=std::string
        ]
note: see reference to class template instantiation 'google::protobuf::internal::MapEntryFuncs<Key,Value,google::protobuf::internal::WireFormatLite::TYPE_STRING,google::protobuf::internal::WireFormatLite::TYPE_BYTES>' being compiled
        with
        [
            Key=std::string,
            Value=std::string
        ]