[arabic] Disable fallback shaping if HB_NO_OT_SHAPE_COMPLEX_ARABIC_FALLBACK defined

Part of https://github.com/harfbuzz/harfbuzz/issues/1652

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

diff --git a/src/hb-ot-shape-complex-arabic.cc b/src/hb-ot-shape-complex-arabic.cc
index bdebde0..4daf2a6 100644
--- a/src/hb-ot-shape-complex-arabic.cc
+++ b/src/hb-ot-shape-complex-arabic.cc
@@ -383,6 +383,10 @@
 		       hb_font_t *font,
 		       hb_buffer_t *buffer)
 {
+#if defined(HB_NO_OT_SHAPE_COMPLEX_ARABIC_FALLBACK)
+  return;
+#endif
+
   const arabic_shape_plan_t *arabic_plan = (const arabic_shape_plan_t *) plan->data;
 
   if (!arabic_plan->do_fallback)