gpg: Keep temp files when opening images via xdg-open

* g10/photoid.c (get_default_photo_command): Change parameter for
xdg-open.

xdg-open spawns the user's preferred image viewer and then exits.
Therefore we must not remove the temp file when it exits,
otherwise by the time the actual image viewer is started the file
doesn't exist any more.

• Signed-off-by: Nicolas Fella <nicolas.fella@gmx.de>