build: fix wayland-backend distcheck

build: fix wayland-backend distcheck

Fix the following error from 'make distcheck':

  CC       libweston/wayland_backend_la-compositor-wayland.lo
../../libweston/compositor-wayland.c:54:51: fatal error: xdg-shell-unstable-v6-client-protocol.h: No such file or directory
 #include "xdg-shell-unstable-v6-client-protocol.h"

Files generated with the scanner belong in nodist_*_SOURCES, not with
the regular sources.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>