rootston: fix compilation without XWayland support

In case wlroots is not being compiled with XWayland support, we will not
have an xwayland surface in our roots view. While we make sure to pay
attention to that in some places, we are not being consistent and try to
access the xwayland surface in other places. Obviously, this leads to a
compiler error due to the field not being present.

Fix the issue by sprinkling in a few additional ifdefs where required.

include/rootston/view.h

@@ -51,7 +51,9 @@ struct roots_xwayland_surface {
 enum roots_view_type {
 	ROOTS_WL_SHELL_VIEW,
 	ROOTS_XDG_SHELL_V6_VIEW,
+#ifdef WLR_HAS_XWAYLAND
 	ROOTS_XWAYLAND_VIEW,
+#endif
 };
 
 struct roots_view {