scanner: Print filename on DTD validation error
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 <67dcd8a17e758192e0cbb577ce1d62cc7da6cf49@gmail.com>
Reviewed-by: David Fort <1a73af9e7ae00182733b2292511b814be66f065f@hardening-consulting.com>
Reviewed-by: Derek Foreman <2f97e0f0bc172406aab8ccd42dab54c95bab8759@osg.samsung.com>
Reviewed-by: Bryce Harrington <bdb14cfc1ab8d24e13de84adc40b41ae60c13bd1@osg.samsung.com>

mchalupa/wayland,sir-murray/wayland,mchalupa/wayland,sir-murray/wayland,sir-murray/wayland,mchalupa/wayland