scanner: Reword fallthrough comment to quiet GCC

GCC 7 now requires an explicit comment noting that case statements
without a break fall through. We already had one of those in the
scanner, but GCC wasn't smart enough to pick it up.

Quiet the warning by making the comment less elaborate.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>