scanner: Print filename on DTD validation error

Don't just print prefix the errors with "protocol", but the actual file
name, if wayland-scanner was passed with the filename of the protocol
file. If wayland-scanner is reading from stdin, errors will be prefixed
with "<stdin>" instead of "protocol".

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: David Fort <contact@hardening-consulting.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>