[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>

fs/gfs2/glock.h

@@ -10,6 +10,8 @@
 #ifndef __GLOCK_DOT_H__
 #define __GLOCK_DOT_H__
 
+#include "incore.h"
+
 /* Flags for lock requests; used in gfs2_holder gh_flag field.
    From lm_interface.h:
 #define LM_FLAG_TRY		0x00000001