Add inline for ODR compliance.

> libSocket.a(Socket.cpp.o): In function `std::iterator_traits<char*>::iterator_category std::__iterator_category<char*>(char* const&)`:
**/usr/local/include/msgpack/object.hpp:472**: multiple definition of `msgpack::operator<<(std::ostream&, msgpack::object const&)`
CMakeFiles/cvim.dir/main.cpp.o:**/usr/local/include/msgpack/object.hpp:472**: first defined here
collect2: error: ld returned 1 exit status

^ Fixes this bug from my project ^