Add missing sig file

ksss · ksss · commit 94914765954b · 2026-03-04T23:23:22.000+09:00
diff --git a/bigdecimal.gemspec b/bigdecimal.gemspec
@@ -34,6 +34,7 @@ Gem::Specification.new do |s|
     sample/linear.rb
     sample/nlsolve.rb
     sample/pi.rb
+    sig/big_decimal_util.rbs
     sig/big_decimal.rbs
     sig/big_math.rbs
   ]
