libweston: fix weston_touch_start_grab() arg name

compositor.c has 'touch', so use 'touch' here as well. It is not a
device to begin with.

Signed-off-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
v1 Tested-by: Matt Hoosier <matt.hoosier@gmail.com>

libweston/compositor.h

@@ -559,7 +559,7 @@ void
 weston_touch_set_focus(struct weston_touch *touch,
 		       struct weston_view *view);
 void
-weston_touch_start_grab(struct weston_touch *device,
+weston_touch_start_grab(struct weston_touch *touch,
 			struct weston_touch_grab *grab);
 void
 weston_touch_end_grab(struct weston_touch *touch);