Qualify call to as_const

[danieljames]

std::as_const was added to C++17 which makes the call ambiguous.

You can see the error at https://www.boost.org/development/tests/develop/developer/output/teeks99-02-dc5-1z-lc-Docker-64on64-boost-bin-v2-libs-graph-test-test_graphs-test-clang-linux-5-0~c++1z~lc-debug.html

In file included from ../libs/graph/test/test_graphs.cpp:18:
In file included from ../libs/graph/test/test_graph.hpp:95:
../libs/graph/test/test_properties.hpp:24:28: error: call to 'as_const' is ambiguous
GraphBundle const& cb1 = as_const(g)[graph_bundle];
                        ^~~~~~~~
../libs/graph/test/test_properties.hpp:120:3: note: in instantiation of function template specialization 'test_graph_bundle<boost::adjacency_matrix<boost::directedS, VertexBundle, EdgeBundle, GraphBundle, std::__1::allocator<bool> > >' requested here
test_graph_bundle(g, graph_bundled);
^
../libs/graph/test/test_graph.hpp:133:5: note: in instantiation of function template specialization 'test_properties<boost::adjacency_matrix<boost::directedS, VertexBundle, EdgeBundle, GraphBundle, std::__1::allocator<bool> >, std::__1::vector<unsigned long, std::__1::allocator<unsigned long> > >' requested here
    test_properties(g, verts);
    ^
../libs/graph/test/test_graphs.cpp:132:5: note: in instantiation of function template specialization 'test_graph<boost::adjacency_matrix<boost::directedS, VertexBundle, EdgeBundle, GraphBundle, std::__1::allocator<bool> > >' requested here
    test_graph(g);
    ^
/usr/include/c++/v1/utility:291:50: note: candidate function [with _Tp = boost::adjacency_matrix<boost::directedS, VertexBundle, EdgeBundle, GraphBundle, std::__1::allocator<bool> >]
template <class _Tp> constexpr add_const_t<_Tp>& as_const(_Tp& __t) noexcept { return __t; }
                                                ^
../libs/graph/test/test_properties.hpp:12:31: note: candidate function [with T = boost::adjacency_matrix<boost::directedS, VertexBundle, EdgeBundle, GraphBundle, std::__1::allocator<bool> >]
template<typename T> T const& as_const(T& x) { return x; }

[danieljames]

Sorry, duplicate of #85.