[GFS2] proper extern for gfs2/locking/dlm/mount.c:gdlm_ops

This patch adds a proper extern declaration for gdlm_ops in
fs/gfs2/locking/dlm/lock_dlm.h

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>