if_enc(4): Make enc_add_hhooks() void

As for the consumer enc_add_hhooks(), hhook_add_hook() will never
fail for the given parameters. Meanwhile, to build the module if_enc(4),
at least option INET or INET6 is required, so no need for the error
EPFNOSUPPORT.

No functional change intended.

Reviewed by: ae
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D46770