net/samba420: Fix missing build dependency of bison

Bison is no longer an optional dependency in Samba 4.20,
so enable it by default. The bison dependency comes from
the Windows Search support implemented in Samba 4.20.

PR: 285180
Approved by: 0mp (mentor)
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D51104