Unbreak the build with (non-default) DNSTAP option set. The root cause
here is that `kdig_CPPFLAGS += $(DNSTAP_CFLAGS)' line is missing from the
src/Makefile.am' file (and thus pre-generated src/Makefile.in') which
should contain "$libfstrm_CFLAGS $libprotobuf_c_CFLAGS" set by configure
script.

As I'd rather avoid patching `src/Makefile.in' and don't want to request
regeneration thereof, provide DNSTAP_CPPFLAGS explicitly for the moment,
while ideally this bug should be fixed upstream. Add an accompanying XXX
comment about it.

PR: 203931 (partially)