Enable DNSTAP where fstrm and protobuf-c are present
The idea was sparked at https://gitlab.isc.org/isc-private/bind-qa/-/issues/38#note_187541.
Prerequisite for bind9!4618 (merged).
Platforms without DNSTAP support (once merged): CentOS 6 (i386, amd64), OpenBSD, and whatever platform we decide to
--disable-dnstap (bind9!4618 (merged) picks Ubuntu Xenial and TSAN jobs due to bind9#1505 (closed)).