Add MVFST_EXPERIMENTAL to server versions.
Add MVFST_EXPERIMENTAL to server versions.

Summary: I always forget this one.

Reviewed By: lnicco

Differential Revision: D23664647

fbshipit-source-id: d9e8e33074c00510ed3b3ae317f6f8674b26a558

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst