bump coreos/go-systemd v19, coreos/pkg v4, godbus/dbus v5.0.1

- https://github.com/coreos/go-systemd/compare/v14..v19
  - coreos/go-systemd#248 dbus: add SetPropertiesSubscriber method
  - coreos/go-systemd#251 activation: add support for listeners with names
  - coreos/go-systemd#296 dbus: Fix API break from godbus
- coreos/pkg@v3...v4
  - no changes in vendored code
- https://github.com/godbus/dbus/compare/v3..v5.0.1
  - godbus/dbus#89 introduce MakeVariantWithSignature

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

vendor.conf

@@ -11,9 +11,9 @@ github.com/sirupsen/logrus 1.0.2
 github.com/syndtr/gocapability d98352740cb2c55f81556b63d4a1ec64c5a319c2
 github.com/vishvananda/netlink 1e2e08e8a2dcdacaae3f14ac44c5cfa31361f270
 # systemd integration.
-github.com/coreos/go-systemd v14
-github.com/coreos/pkg v3
-github.com/godbus/dbus v3
+github.com/coreos/go-systemd v19
+github.com/coreos/pkg v4
+github.com/godbus/dbus v5.0.1
 github.com/golang/protobuf 18c9bb3261723cd5401db4d0c9fbc5c3b6c70fe8
 # Command-line interface.
 github.com/cyphar/filepath-securejoin v0.2.2