knot-resolver: enable dnstap module build by default

'dnstap' module will be built but not loaded by default at runtime
(configuration must be provided for it to be loaded). It is still possible to
disable dnstap build manually using menuconfig.

"The dnstap module supports logging DNS requests and responses to a unix socket
in dnstap format using fstrm framing library. This logging is useful if you need
effectively log all DNS traffic."[^1]

Adds dependency on 'protobuf', 'protobuf-c', 'libfstrm'. Listed packages are
available from OpenWrt packages, have uncomplicated manifests and
while 'protobuf-c' doesn't have a maintainer since spring 2020, all the
packages (including 'protobuf-c') seem to be maintained - the last
updates of all of them in autumn 2021.

As stated by Vladimír Čunát from Knot Resolver team they build dnstap
while packaging for majority of standard Linux distributions.
Therefore this change brings us closer to expected default.

[^1]: https://knot-resolver.readthedocs.io/en/stable/modules-dnstap.html