xen: make blkif_getgeo static

Introduced between 2.6.25-rc2 and -rc3
drivers/block/xen-blkfront.c:139:5: warning: symbol 'blkif_getgeo' was not declared. Should it be static?

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>