tuntap(4): properly declare if_tun and if_tap modules

Simply adding MODULE_VERSION does not do the trick, because the modules
haven't been declared. This should actually fix modfind/kldstat, which
r351229 aimed and failed to do.

This should make vm-bhyve do the right thing again when using the ports
version, rather than the latest version not in ports.

MFC after: 3 days