Fix warnings

src/hb-ot-shape-complex-arabic-fallback.hh

diff --git a/src/hb-ot-shape-complex-arabic-fallback.hh b/src/hb-ot-shape-complex-arabic-fallback.hh
index bf68561..4fcd0a2 100644
--- a/src/hb-ot-shape-complex-arabic-fallback.hh
+++ b/src/hb-ot-shape-complex-arabic-fallback.hh
@@ -53,7 +53,7 @@
 };
 
 static OT::SubstLookup *
-arabic_fallback_synthesize_lookup_single (const hb_ot_shape_plan_t *plan,
+arabic_fallback_synthesize_lookup_single (const hb_ot_shape_plan_t *plan HB_UNUSED,
 					  hb_font_t *font,
 					  unsigned int feature_index)
 {
@@ -103,7 +103,7 @@
 }
 
 static OT::SubstLookup *
-arabic_fallback_synthesize_lookup_ligature (const hb_ot_shape_plan_t *plan,
+arabic_fallback_synthesize_lookup_ligature (const hb_ot_shape_plan_t *plan HB_UNUSED,
 					    hb_font_t *font)
 {
   OT::GlyphID first_glyphs[ARRAY_LENGTH_CONST (ligature_table)];