Add msys2 library dependency tag in gem metadata

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

This fixes issues like
oneclick/rubyinstaller2#54 and
oneclick/rubyinstaller2#53 .

Author: larskanis

openssl.gemspec

@@ -21,4 +21,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rake-compiler"
   spec.add_development_dependency "test-unit", "~> 3.0"
   spec.add_development_dependency "rdoc"
+
+  spec.metadata["msys2_mingw_dependencies"] = "openssl"
 end