[fix] Added difference_type for ImmutableTree::iterator

[S1eGa]

For configuration

clang version 10.0.0-4ubuntu1
Target: x86_64-pc-linux-gnu
Thread model: posix

build fails with error error: no type named `difference_type' in `struct std::iterator_traits<klee::ImmutableTree<klee::ref<klee::Target>, klee::ref<klee::Target>, klee::_Identity<klee::ref<klee::Target>>, std::less<klee::ref<klee::Target>>>::iterator>'on this line:

DistanceResult stateDistance = distance(state, target);

Problem resolves with addition of using declarations in iterator declaration.