build: move wayland-client dep to backend/wayland/

wayland-client isn't really used by wlroots core, so let's move the
dep to where it's needed in the Wayland backend.

backend/wayland/meson.build

@@ -1,3 +1,9 @@
+wayland_client = dependency('wayland-client',
+	fallback: ['wayland', 'wayland_client_dep'],
+	default_options: wayland_project_options,
+)
+wlr_deps += wayland_client
+
 wlr_files += files(
 	'backend.c',
 	'output.c',