meson: allow to build weston-simple-dmabuf-egl

It is all hooked up in clients/meson.build, just the option to enable it
was missing.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

meson_options.txt

@@ -180,7 +180,7 @@ option(
 option(
 	'simple-clients',
 	type: 'array',
-	choices: [ 'all', 'damage', 'im', 'egl', 'shm', 'touch', 'dmabuf-v4l' ],
+	choices: [ 'all', 'damage', 'im', 'egl', 'shm', 'touch', 'dmabuf-v4l', 'dmabuf-egl' ],
 	value: [ 'all' ],
 	description: 'Sample clients: simple test programs'
 )