mruby-io: restore Windows link library dropped during HAL migration

Commit 9965f11cfe1c ("mruby-io: migrate HAL to ports/ directories")
moved the Windows HAL source into mruby-io/ports/win/ but dropped the
linker.libraries declaration that previously lived in
hal-win-io/mrbgem.rake. Both mingw and MSVC builds now fail to link
with undefined references to closesocket, WSAStartup, WSACleanup,
select, and WSAGetLastError referenced from io.c and io_hal.c.
Re-declare ws2_32 under a for_windows? guard.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-io/mrbgem.rake

@@ -5,4 +5,8 @@ MRuby::Gem::Specification.new('mruby-io') do |spec|
 
   spec.build.defines << "HAVE_MRUBY_IO_GEM"
   spec.add_test_dependency 'mruby-time', core: 'mruby-time'
+
+  if spec.for_windows?
+    spec.linker.libraries << "ws2_32"
+  end
 end