Add missing $PATCH_DIR prefix for WebRtc patch.

It was missing in ac95a438620f3ec7c72ae60396ee26b3778d38b2.

Change-Id: Ib08ea6466835e9d2c55bc7944ae1b758370da30a
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>