fujitsu-laptop: fix compiler warning on pnp_ids

Annotate pnp_ids as '__used' to fix following warning:

  CC      drivers/platform/x86/fujitsu-laptop.o
drivers/platform/x86/fujitsu-laptop.c:1243: warning: ‘pnp_ids’ defined but not used

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>