SUNRPC: Fix the NFSv4 and RPCSEC_GSS Kconfig dependencies

The NFSv4 client's callback server calls svc_gss_principal(), which
is defined in the auth_rpcgss.ko

The NFSv4 server has the same dependency, and in addition calls
svcauth_gss_flavor(), gss_mech_get_by_pseudoflavor(),
gss_pseudoflavor_to_service() and gss_mech_put() from the same module.

The module auth_rpcgss itself has no dependencies aside from sunrpc,
so we only need to select RPCSEC_GSS.

Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>