devel/hapy: Fix build with Clang 6

In file included from Algorithm.cc:4:
In file included from ../src/include/Hapy/Algorithm.h:6:
In file included from ../src/include/Hapy/Result.h:7:
In file included from ../src/include/Hapy/Pree.h:9:
In file included from ../src/include/Hapy/PreeKids.h:7:
../src/include/Hapy/Iterator.h:22:10: error: no template named 'bidirectional_iterator' in namespace 'std'; did you mean 'std_bidirectional_iterator'?

public std::bidirectional_iterator<ValueT, DiffT> {
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~
       std_bidirectional_iterator

etc.

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/hapy-0.0.8_1.log

PR: 230907
Submitted by: tobik
Approved by: Alex Rousskov <rousskov@measurement-factory.com> (maintainer)