High number of warnings being emitted from phosphor on windows

when we're building shared objects we need to add the declspec attribute on all of the template stuff we're using:

c:\jenkins\workspace\memcached-gerrit-master\label\windows\phosphor\include\phosphor\trace_log.h(448) : warning C4251: 'phosphor::TraceLog::thread_names' : class 'std::unordered_map<uint64_t,std::string,std::hash<unsigned __int64>,std::equal_to<_Kty>,std::allocator<std::pair<const _Kty,_Ty>>>' needs to have dll-interface to be used by clients of class 'phosphor::TraceLog'

See https://support.microsoft.com/en-us/kb/168958