Improve syntax to make out linux-arm64 a little happy

Decided to apply is we did the same on other places however this won't
fix all of its complains

src/hb-subset-plan.cc

@@ -179,7 +179,7 @@ _create_old_gid_to_new_gid_map (const hb_face_t *face,
 
     unsigned max_glyph =
     + hb_iter (all_gids_to_retain)
-    | hb_reduce (hb_max, 0)
+    | hb_reduce (hb_max, 0u)
     ;
     *num_glyphs = max_glyph + 1;
   }