fix portable compilation issue

Closes #2348 (closed) yet again.

On some systems, unit test fail to build with:

allocation_state_unittest.cc:35:5:   required from here
 /usr/src/googletest/googletest/include/gtest/gtest.h:1392:11: error: no match for ‘operator==’ (operand types are ‘const boost::date_time::special_values’ and ‘const boost::posix_time::ptime’)
    if (lhs == rhs) {
        ~~~~^~~~~~

Latest job with error: https://jenkins.aws.isc.org/job/kea-dev/job/ut-extended/912/

Job on this branch: https://jenkins.aws.isc.org/view/Kea-manual/job/kea-manual/job/ut-extended/167/