Build failure using mingw

[fundamental]

When cross compiling with mingw mruby-io adds 'Ws2_32' to the list of libraries to be linked to: https://github.com/iij/mruby-io/blob/master/mrbgem.rake#L7-L12

mingw however does not have this library, but it does have 'ws2_32' which is included later in the same gem file.

To build the full project that I'm working with RUBY_PLATFORM needs to be 'mingw' as otherwise mruby-regexp-pcre fails to build. This leads me to think that https://github.com/iij/mruby-io/blob/master/mrbgem.rake#L8 should read when /mswin/ instead of when /mingw|mswin/.