color: declare cm_image_desc_info

Fixes compiler warnings about the declaration in function signatures not
applying outside of the functions.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

libweston/color-management.h

@@ -28,6 +28,8 @@
 
 #include "color-properties.h"
 
+struct cm_image_desc_info;
+
 int
 weston_compositor_enable_color_management_protocol(struct weston_compositor *compositor);