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_unimplemented.go

@@ -0,0 +1,14 @@
+// +build !darwin,!linux,!windows
+
+package gateway
+
+import (
+	"fmt"
+	"net"
+	"runtime"
+)
+
+func DiscoverGateway() (ip net.IP, err error) {
+	err = fmt.Errorf("DiscoverGateway not implemented for OS %s", runtime.GOOS)
+	return
+}