add key_revoke() dummy for KEYS=n

This fixes the following build error with CONFIG_KEYS=n, caused by
commit dfd15c46

 ("cifs: explicitly
revoke SPNEGO key after session setup"):

    CC [M]  fs/cifs/sess.o
  fs/cifs/sess.c: In function 'CIFS_SessSetup':
  fs/cifs/sess.c:628: error: implicit declaration of function 'key_revoke'
  make[3]: *** [fs/cifs/sess.o] Error 1

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>