modules/input: don't dist generated protocol sources

For some reason modules/input adds the C code that is generated by wayland-scanner to EXTRA_DIST. This results in a race where depending on the timestamps of the tarball and the timestamp of the system wayland-protocols either the generated source in the tarball or a regenerated source will be used.

This doesn't actually impact the code generated, but it does break reproducible builds as the list of source files changes.