Add msys2 library dependency tag in gem metadata.

RubyInstaller2 supports metadata tags for installation of dependent
MSYS2/MINGW libraries. The fxruby source gem requires the fox
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