scripts/gdb: enable completion for lx-list-check parameter

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Thiébaud Weksteen <thiebaud@weksteen.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>