Fix error in lib testutils detected by UBSan

There is linking issue detected by UBSan in freebsd-13.0-amd64 build and fedora-38-amd64-clang build stages:

11:14:27  ld: error: undefined symbol: typeinfo for isc::dhcp::Pkt
11:14:27  >>> referenced by log_utils.cc
11:14:27  >>>               libkea_testutils_la-log_utils.o:(.data+0x1558) in archive ../../../../src/lib/testutils/.libs/libkea-testutils.a
11:14:27  c++: error: linker command failed with exit code 1 (use -v to see invocation)
11:14:27  *** [libhttp_unittests] Error code 1

This UB was introduced in #2820 (closed).