rocket: fix compiler warning on rocket_pci_ids

Annotate rocket_pci_ids as '__used' to fix following warning:

  CC      drivers/char/rocket.o
drivers/char/rocket.c:1767: warning: ‘rocket_pci_ids’ defined but not used

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>