nfsd: fix dependency of nfsd on auth_rpcgss

Commit b0b0c0a2 "nfsd: add proc file listing kernel's gss_krb5
enctypes" added an nunnecessary dependency of nfsd on the auth_rpcgss
module.

It's a little ad hoc, but since the only piece of information nfsd needs
from rpcsec_gss_krb5 is a single static string, one solution is just to
share it with an include file.

Cc: stable@kernel.org
Reported-by: Michael Guntsche <mike@it-loops.com>
Cc: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>