evdev: Add function to get the libinput_device from a ClutterInputDevice

This may be useful for deeper libinput integration that's not in the scope
of Clutter.

https://bugzilla.gnome.org/show_bug.cgi?id=728967

clutter/evdev/clutter-evdev.h

@@ -28,6 +28,7 @@
 #include <glib-object.h>
 #include <xkbcommon/xkbcommon.h>
 #include <clutter/clutter.h>
+#include <libinput.h>
 
 G_BEGIN_DECLS
 
@@ -112,6 +113,8 @@ void clutter_evdev_add_filter    (ClutterEvdevFilterFunc func,
 CLUTTER_AVAILABLE_IN_1_20
 void clutter_evdev_remove_filter (ClutterEvdevFilterFunc func,
                                   gpointer               data);
+CLUTTER_AVAILABLE_IN_1_20
+struct libinput_device * clutter_evdev_input_device_get_libinput_device (ClutterInputDevice *device);
 
 G_END_DECLS