Backport an upstream commit that fixes the build with libc++ 3.9.0.

Ever since LLVM 3.9.0 was imported into base, qbs was failing to build with an
error message like this:

In file included from /wrkdirs/usr/ports/devel/qbs/work/qbs-src-1.6.1/src/lib/corelib/api/internaljobs.cpp:39:
In file included from /wrkdirs/usr/ports/devel/qbs/work/qbs-src-1.6.1/src/lib/corelib/buildgraph/executor.h:45:
/usr/include/c++/v1/queue:400:5: error: static_assert failed ""

static_assert((is_same<_Tp, value_type>::value), "" );
^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/wrkdirs/usr/ports/devel/qbs/work/qbs-src-1.6.1/src/lib/corelib/buildgraph/executor.h:156:12: note: in instantiation of template class 'std::1::priority_queue<qbs::Internal::Artifact *, std::1::vector<qbs::Internal::BuildGraphNode *, std::__1::allocator<qbs::Internal::BuildGraphNode *> >, qbs::Internal::Executor::ComparePriority>' requested here

Leaves m_leaves;

I don't think PORTREVISION needs to be bumped, as there shouldn't be any
difference on systems where the port was already building fine.

MFH: 2016Q4