libceph: make cancel_generic_request() static

Fixes the following sparse warning:

net/ceph/mon_client.c:577:6: warning:
 symbol 'cancel_generic_request' was not declared. Should it be static?
Signed-off-by: NWei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

net/ceph/mon_client.c

@@ -574,7 +574,7 @@ static void complete_generic_request(struct ceph_mon_generic_request *req)
 	put_generic_request(req);
 }
 
-void cancel_generic_request(struct ceph_mon_generic_request *req)
+static void cancel_generic_request(struct ceph_mon_generic_request *req)
 {
 	struct ceph_mon_client *monc = req->monc;
 	struct ceph_mon_generic_request *lookup_req;