ceph: add missing #includes

Signed-off-by: Sage Weil <sage@newdream.net>
diff --git a/fs/ceph/super.h b/fs/ceph/super.h
index e30dfbb..13513b8 100644
--- a/fs/ceph/super.h
+++ b/fs/ceph/super.h
@@ -10,6 +10,7 @@
 #include <linux/fs.h>
 #include <linux/mempool.h>
 #include <linux/pagemap.h>
+#include <linux/slab.h>
 #include <linux/wait.h>
 #include <linux/writeback.h>
 #include <linux/slab.h>