vendor: Replace github.com/jackpal/gateway with github.com/calmh/gateway (fixes #3142)

Switch to my forked version which contains a fix for this issue. I'll
track upstream in the future if things update there, and attempt to
contribute back fixes...

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3149

vendor/github.com/calmh/gateway/gateway_windows.go

@@ -0,0 +1,16 @@
+package gateway
+
+import (
+	"net"
+	"os/exec"
+)
+
+func DiscoverGateway() (ip net.IP, err error) {
+	routeCmd := exec.Command("route", "print", "0.0.0.0")
+	output, err := routeCmd.CombinedOutput()
+	if err != nil {
+		return nil, err
+	}
+
+	return parseRoutePrint(output)
+}