Fix warnings

src/hb-ot-shape-fallback.cc

diff --git a/src/hb-ot-shape-fallback.cc b/src/hb-ot-shape-fallback.cc
index f4463e4..6f3426e 100644
--- a/src/hb-ot-shape-fallback.cc
+++ b/src/hb-ot-shape-fallback.cc
@@ -161,8 +161,8 @@
 }
 
 void
-_hb_ot_shape_fallback_position_recategorize_marks (const hb_ot_shape_plan_t *plan,
-						   hb_font_t *font,
+_hb_ot_shape_fallback_position_recategorize_marks (const hb_ot_shape_plan_t *plan HB_UNUSED,
+						   hb_font_t *font HB_UNUSED,
 						   hb_buffer_t  *buffer)
 {
   unsigned int count = buffer->len;