defines.hpp missing keyword inline on operator << in MSGPACK_ADD_ENUM Macro

[rkloeters]

When trying to use the Macro MSGPACK_ADD_ENUM on a baseclass for derived classes that use the same enum in the derived class causes a linker error "Multiple definition of operator <<".
The operator << should be using the keyword inline too.

[redboltz]

Thank you for reporting. I sent a pull request to fix it.