mruby-task: link with winmm library on windows

windows multimedia timer api requires linking with winmm.lib. added
conditional linker library using spec.for_windows? to match mruby
build system conventions.

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

mrbgems/mruby-task/mrbgem.rake

@@ -6,5 +6,10 @@ MRuby::Gem::Specification.new('mruby-task') do |spec|
   # Enable task scheduler globally (required for vm.c integration)
   spec.build.defines << 'MRB_USE_TASK_SCHEDULER'
 
+  # Windows: link with multimedia timer library
+  if spec.for_windows?
+    spec.linker.libraries << 'winmm'
+  end
+
   spec.add_dependency 'mruby-fiber'  # Uses same context infrastructure
 end