Fix warnings

src/hb-ot-shape-complex-thai.cc

diff --git a/src/hb-ot-shape-complex-thai.cc b/src/hb-ot-shape-complex-thai.cc
index 0760891..24d476a 100644
--- a/src/hb-ot-shape-complex-thai.cc
+++ b/src/hb-ot-shape-complex-thai.cc
@@ -215,7 +215,7 @@
 
 
 static void
-do_thai_pua_shaping (const hb_ot_shape_plan_t *plan,
+do_thai_pua_shaping (const hb_ot_shape_plan_t *plan HB_UNUSED,
 		     hb_buffer_t              *buffer,
 		     hb_font_t                *font)
 {