backends: Get rid of meta-cursor-private.h

There is nothing special about the private API which only consists of
getters for renderer specific backing buffer. Lets them to the regular
.h file and treat them as part of the normal API.

https://bugzilla.gnome.org/show_bug.cgi?id=744932

src/backends/native/meta-cursor-renderer-native.c

@@ -29,7 +29,6 @@
 #include <gbm.h>
 #include <xf86drm.h>
 
-#include "meta-cursor-private.h"
 #include "meta-monitor-manager-private.h"
 
 #ifndef DRM_CAP_CURSOR_WIDTH