touch: Make weston_touch_set_focus() take a touch instead of a seat

The other set_focus() functions take the relevant type instead of a seat
already, so this is consistent.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
diff --git a/src/compositor.h b/src/compositor.h
index 31ef5c7..20109d7 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -418,7 +418,7 @@
 void
 weston_touch_destroy(struct weston_touch *touch);
 void
-weston_touch_set_focus(struct weston_seat *seat,
+weston_touch_set_focus(struct weston_touch *touch,
 		       struct weston_view *view);
 void
 weston_touch_start_grab(struct weston_touch *device,