[GFS2] Make headers compile on their own

As per Jan Engelhardt's comments, this should make all the headers
compile on their own by including and/or declaring structures
early.

Cc: Jan Engelhardt <jengelh@linux01.gwdg.de>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
diff --git a/fs/gfs2/acl.h b/fs/gfs2/acl.h
index 5856ba7..05c294f 100644
--- a/fs/gfs2/acl.h
+++ b/fs/gfs2/acl.h
@@ -10,6 +10,8 @@
 #ifndef __ACL_DOT_H__
 #define __ACL_DOT_H__
 
+#include "incore.h"
+
 #define GFS2_POSIX_ACL_ACCESS		"posix_acl_access"
 #define GFS2_POSIX_ACL_ACCESS_LEN	16
 #define GFS2_POSIX_ACL_DEFAULT		"posix_acl_default"