xwayland: Introduce a private struct for XWayland interface

libweston-desktop implements this private struct.

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Jonas Ã…dahl <jadahl@gmail.com>
Acked-by: Giulio Camuffo <giulio.camuffo@kdab.com>

Differential Revision: https://phabricator.freedesktop.org/D1208
diff --git a/Makefile.am b/Makefile.am
index 217d21f..e7a76c1 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1028,6 +1028,7 @@
 	$(CAIRO_CFLAGS)
 xwayland_la_SOURCES =				\
 	xwayland/xwayland.h			\
+	xwayland/xwayland-internal-interface.h	\
 	xwayland/window-manager.c		\
 	xwayland/selection.c			\
 	xwayland/dnd.c				\