Fix warning on GCC.
Fix warning on GCC.

Warning fixed:
luabind/detail/policy.hpp:210: warning: type qualifiers ignored on function return type

Oberon00/luabind,Oberon00/luabind,rpavlik/luabind,rpavlik/luabind,Kazade/luabind-deboostified,Wohlhabend-Networks/luabind-deboostified,rpavlik/luabind,ltjax/luabind-deboostified,MrApplejuice/luabind,decimad/luabind-deboostified,rpavlik/luabind,grafi-tt/luabind,davidar/luabind,plsytj/luabind,alex85k/luabind,lonski/luabind,rpavlik/luabind,galek/luabind-deboostified,alex85k/luabind,alex85k/luabind,mapbox/luabind,alex85k/luabind