rsocket: avoid link issues with gcc7 and gold
rsocket: avoid link issues with gcc7 and gold

Summary:
Remove the constexpr from the definition of the static ProtocolVersion::Unknown member
to ensure it's made available to the linker.

Reviewed By: yfeldblum

Differential Revision: D6780586

fbshipit-source-id: 2bb9752b33433d4aef21da9f270b92a5fa8a9abd

rsocket/rsocket-cpp,phoad/rsocket-cpp,phoad/rsocket-cpp,phoad/rsocket-cpp,rsocket/rsocket-cpp,phoad/rsocket-cpp,rsocket/rsocket-cpp,phoad/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,ReactiveSocket/reactivesocket-cpp,ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp