scd: Add 0x41 prefix for x-coordinate only result.

* scd/app-openpgp.c (do_decipher): When it's x-coordinate only, add the
prefix 0x41.

Card should return fixed size bytes, either in format of
(04 || X || Y) or (X, x-coordinate only).

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>