wg: Implement if_transmit unconditionally

Commit bf454ca88bdf made wg_transmit() defined only when "device netmap"
is configured, as if_wg's if_transmit implementation should never be
called otherwise, but this breaks a requirement that interfaces
implement both or neither of if_transmit and if_qflush.

Restore the old behaviour of unconditionally defining wg_transmit(). It
contains an assertion that the interface is in netmap mode.

Reported by: peterj
MFC after: 2 weeks
Fixes: bf454ca88bdf ("wg: Add netmap support")