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>

Makefile.am

@@ -394,12 +394,12 @@ wayland_backend_la_CFLAGS =			\
 wayland_backend_la_SOURCES = 					\
 	libweston/compositor-wayland.c				\
 	libweston/compositor-wayland.h				\
-	protocol/xdg-shell-unstable-v6-protocol.c		\
-	protocol/xdg-shell-unstable-v6-client-protocol.h	\
 	shared/helpers.h
 nodist_wayland_backend_la_SOURCES =				\
 	protocol/fullscreen-shell-unstable-v1-protocol.c	\
-	protocol/fullscreen-shell-unstable-v1-client-protocol.h
+	protocol/fullscreen-shell-unstable-v1-client-protocol.h	\
+	protocol/xdg-shell-unstable-v6-protocol.c		\
+	protocol/xdg-shell-unstable-v6-client-protocol.h
 endif
 
 if ENABLE_HEADLESS_COMPOSITOR