Add set_system_proxy for linux and android

common/settings/proxy_stub.go

@@ -0,0 +1,14 @@
+//go:build !windows && !linux
+
+package settings
+
+import "github.com/sagernet/sing-box/log"
+
+func ClearSystemProxy() error {
+	return nil
+}
+
+func SetSystemProxy(port uint16, mixed bool) error {
+	log.Warn("set system proxy: unsupported operating system")
+	return nil
+}