test: fix ambiguous reference to _1

test: fix ambiguous reference to _1

FreeBSD/Clang/lib complains:
/home/jenkins/workspace/ceph-master/src/test/librados_test_stub/TestRadosClient.cc:295:55: error: reference to '_1' is ambiguous
    &TestRadosClient::finish_aio_completion, this, c, _1));
                                                      ^
/usr/local/include/boost/bind/placeholders.hpp:46:38: note: candidate found by name lookup is 'boost::placeholders::_1'
BOOST_STATIC_CONSTEXPR boost::arg<1> _1;
                                     ^
/usr/include/c++/v1/functional:2448:46: note: candidate found by name lookup is 'std::__1::placeholders::_1'
/* _LIBCPP_INLINE_VAR */ constexpr __ph<1>   _1{};

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>