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

Commit be6413f0d86a ("mruby-task: migrate HAL to ports/ directories")
moved the Windows HAL source into mruby-task/ports/win/ but dropped
the linker.libraries declaration that previously lived in
hal-win-task/mrbgem.rake. Both mingw and MSVC builds now fail to link
task_hal.o/obj with undefined references to timeBeginPeriod,
timeEndPeriod, timeSetEvent, and timeKillEvent. Re-declare winmm
under a for_windows? guard.

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

mrbgems/mruby-task/mrbgem.rake

@@ -5,4 +5,8 @@ MRuby::Gem::Specification.new('mruby-task') do |spec|
 
   # Enable task scheduler globally (required for vm.c integration)
   spec.build.defines << 'MRB_USE_TASK_SCHEDULER'
+
+  if spec.for_windows?
+    spec.linker.libraries << "winmm"
+  end
 end