Remove boost placeholders from global namespace

[phako]

With newer boost this is deprecated and throws a ton of warnings:

In file included from /usr/include/boost/bind.hpp:30,
                 from /home/jgeorg/jhbuild/checkout/DeviceAccess/util/include/VirtualFunctionTemplate.h:26,
                 from /home/jgeorg/jhbuild/checkout/DeviceAccess/device_backends/include/DeviceBackend.h:17,
                 from /home/jgeorg/jhbuild/checkout/DeviceAccess/device_backends/src/DeviceBackend.cc:5:
/usr/include/boost/bind.hpp:36:1: note: ‘#pragma message: The practice of declaring the Bind placeholders (_1, _2, ...) in the global namespace is deprecated. Please use <boost/bind/bind.hpp> + using namespace boost::placeholders, or define BOOST_BIND_GLOBAL_PLACEHOLDERS to retain the current behavior.’