At dist/distcheck time we need to ensure that all the files and their
respective dependencies are handled.

At the moment we'll bail out as the linux-dmabuf rules are guarded in a
conditional. Move them outside of it and drop the sources from
BUILT_SOURCES.

Thus the files will be generated only as needed, which will happen only
after the wayland-protocols dependency is enforced in configure.ac.

v2: add dependency tracking for the header

Cc: Andres Gomez <agomez@igalia.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>