[9.20] fix: nil: Fix building on uclibc

While building on uclibc this error is thrown:
In file included from ./include/dns/log.h:20,
                 from callbacks.c:19:
../../lib/isc/include/isc/log.h:141:9: error: unknown type name ‘off_t’
  141 |         off_t maximum_size;
      |         ^~~~~

This is due to missing include unistd.h, so let's add it on top of
isc/log.h

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Backport of MR !11422

Merge branch 'backport-fix/uclibc-off_t-main-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11423