librpcsec_gss: don't export non-existant symbols

rpc_gss_mesh_to_oid was never implemented.

This doesn't seem to be any reason why we would need to export the _stub
functions, but it's also a little unclear to me why the linker thinks
they aren't present. Perhaps they should be static.

Differential Revision: https://reviews.freebsd.org/D44248