loop.h build fix

include/linux/loop.h:66: error: expected specifier-qualifier-list before 'request_queue_t'

Cc: Sebastian Siewior <sebastian@breakpoint.cc>
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>