lib/upnp: Fix build

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4316

lib/upnp/upnp.go

@@ -42,7 +42,6 @@ import (
 	"net"
 	"net/http"
 	"net/url"
-	"regexp"
 	"runtime"
 	"strings"
 	"time"