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.c b/src/compositor.c
index 1c06265..698ee52 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -1814,7 +1814,7 @@
 						 wl_fixed_from_int(0),
 						 wl_fixed_from_int(0));
 		if (seat->touch && seat->touch->focus == view)
-			weston_touch_set_focus(seat, NULL);
+			weston_touch_set_focus(seat->touch, NULL);
 	}
 }