Fix windows linker errors

[multiprogramm]

Problem:

1. Build cppkafka as dll on Windows (MSVS 2015).
2. Link library to the project with examples/buffered_producer_example.cpp for example.
3. Build.

Error	LNK2001	unresolved external symbol "public: class std::shared_ptr<class cppkafka::Internal> __thiscall cppkafka::MessageInternal::get_internal(void)const " (?get_internal@MessageInternal@cppkafka@@QBE?AV?$shared_ptr@VInternal@cppkafka@@@std@@XZ)

Error	LNK2001	unresolved external symbol "public: static class std::unique_ptr<class cppkafka::MessageInternal,struct std::default_delete<class cppkafka::MessageInternal> > __cdecl cppkafka::MessageInternal::load(class cppkafka::Message &)" (?load@MessageInternal@cppkafka@@SA?AV?$unique_ptr@VMessageInternal@cppkafka@@U?$default_delete@VMessageInternal@cppkafka@@@std@@@std@@AAVMessage@2@@Z)

Error	LNK2001	unresolved external symbol "public: __thiscall cppkafka::MessageInternal::MessageInternal(void *,class std::shared_ptr<class cppkafka::Internal>)" (??0MessageInternal@cppkafka@@QAE@PAXV?$shared_ptr@VInternal@cppkafka@@@std@@@Z)

[mfontanini]

Thanks! PR build failed but that's unrelated.