[os2] Minor add a cast

src/hb-ot-os2-unicode-ranges.hh

@@ -223,7 +223,7 @@ static unsigned int
 _hb_ot_os2_get_unicode_range_bit (hb_codepoint_t cp)
 {
   auto *range = hb_sorted_array (_hb_os2_unicode_ranges).bsearch (cp);
-  return range ? range->bit : -1;
+  return range ? range->bit : (unsigned) -1;
 }
 
 } /* namespace OT */