Add missing windows build tag to spawn.go

mattn · mattn · commit ce2849d633fa · 2026-03-21T00:27:57.000+09:00
spawn.go uses Windows-only symbols (_ShellExecuteNowait, makeCmdLine,
syscall.SW_NORMAL) but was missing the build tag, causing compilation
errors on non-Windows platforms.
diff --git a/spawn.go b/spawn.go
@@ -1,3 +1,6 @@
+//go:build windows
+// +build windows
+
 package main
 
 import (
