[ruby/fiddle] Use msys2_mingw_dependencies

https://github.com/ruby/fiddle/commit/fee175a8ff

ext/fiddle/fiddle.gemspec

@@ -65,4 +65,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake"
   spec.add_development_dependency "rake-compiler"
+
+  spec.metadata["msys2_mingw_dependencies"] = "libffi"
 end