Add dependency to gdbm package on mingw

RubyInstaller2 supports metadata tags for installation of dependent MSYS2/MINGW libraries. The openssl gem requires the mingw-openssl package to be installed on the system, which the gem installer takes care about, when this tag is set. The feature is documented here: https://github.com/oneclick/rubyinstaller2/wiki/For-gem-developers#msys2-library-dependency Fixes oneclick/rubyinstaller2#163

Author: larskanis

gdbm.gemspec

@@ -18,4 +18,5 @@ Gem::Specification.new do |spec|
  spec.require_paths = ["lib"]
  spec.extensions = ["ext/gdbm/extconf.rb"]
  spec.required_ruby_version = ">= 2.3.0"
+ spec.metadata["msys2_mingw_dependencies"] = "gdbm"
 end