SLOW_WORK: Fix GFS2 to #include <linux/module.h> before using THIS_MODULE

GFS2 has been altered to pass THIS_MODULE to slow_work_register_user(), but
hasn't been altered to #include <linux/module.h> to provide it, resulting in
the following error:

	fs/gfs2/recovery.c:596: error: 'THIS_MODULE' undeclared here (not in a function)

Add the missing #include.

Signed-off-by: David Howells <dhowells@redhat.com>