ceph: add missing #includes

Signed-off-by: Sage Weil <sage@newdream.net>
diff --git a/fs/ceph/auth.c b/fs/ceph/auth.c
index f6394b9..818afe7 100644
--- a/fs/ceph/auth.c
+++ b/fs/ceph/auth.c
@@ -3,6 +3,7 @@
 #include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/err.h>
+#include <linux/slab.h>
 
 #include "types.h"
 #include "auth_none.h"