mruby-socket: restore Windows link libraries dropped during HAL migration

Commit d9e107c801 ("mruby-socket: migrate HAL to ports/ directories")
moved the Windows HAL source into mruby-socket/ports/win/ but dropped
the linker.libraries declarations that previously lived in
hal-win-socket/mrbgem.rake. Both mingw and MSVC builds now fail to
link with undefined references to WSAStartup, socket, send, and other
Winsock APIs. Re-declare wsock32 and ws2_32 under a for_windows? guard.

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

mrbgems/mruby-socket/mrbgem.rake

@@ -8,4 +8,9 @@ MRuby::Gem::Specification.new('mruby-socket') do |spec|
   spec.add_dependency('mruby-io', :core => 'mruby-io')
   spec.add_dependency('mruby-error', :core => 'mruby-error')
   # spec.add_dependency('mruby-mtest')
+
+  if spec.for_windows?
+    spec.linker.libraries << "wsock32"
+    spec.linker.libraries << "ws2_32"
+  end
 end