selftests: ublk: add missing gitignore for metadata_size binary

A new utility metadata_size was added in
commit 261b67f4e347 ("selftests: ublk: add utility to get block device metadata size")
but it was not added to .gitignore. Fix that by adding it there.

While at it sort all entries alphabetically and add a SPDX license header.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Fixes: 261b67f4e347 ("selftests: ublk: add utility to get block device metadata size")
Signed-off-by: Alexander Atanasov <alex@zazolabs.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>