dbus: Ensure to build with systemd support

Pass `--enable-system` to the configure script to Ensure, that D-Bus is
built with systemd support. By default, the build still succeeds, but
systemd support is disabled, if the configure script doesn’t find the
right version.

dbus.be0

@@ -59,6 +59,7 @@ mee_configure() {
 
     bee_configure \
         --with-dbus-user=dbus \
+        --enable-systemd \
         --with-systemdsystemunitdir=${systemdsystemunitdir}
 }