add key_revoke() dummy for KEYS=n

This fixes the following build error with CONFIG_KEYS=n, caused by
commit dfd15c46a6c2cafb006183c0c14f07e59eee4ac0 ("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 <[e-mail]>
Acked-by: Jeff Layton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/key.h

···
 #define key_validate(k)			0
 #define key_serial(k)			0
 #define key_get(k) 			({ NULL; })
+#define key_revoke(k)			do { } while(0)
 #define key_put(k)			do { } while(0)
 #define key_ref_put(k)			do { } while(0)
 #define make_key_ref(k, p)			({ NULL; })