Move the optional output name property from drm_output to weston_output

In the future the CMS plugins will need to read the config file and setup a list
of hardcoded names to ICC profiles.
diff --git a/src/compositor.h b/src/compositor.h
index 63d1127..7da6c48 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -156,6 +156,7 @@
 
 struct weston_output {
 	uint32_t id;
+	char *name;
 
 	void *renderer_state;