Fix a bug in vex::generator::Kernel::param_size
Fix a bug in vex::generator::Kernel::param_size

Make param_size struct conform to boost::result_of protocol. Without
this, build was broken for compilers without decltype support.

kapadia/vexcl,Poulpy21/vexcl,ddemidov/vexcl,kapadia/vexcl,ddemidov/vexcl,Poulpy21/vexcl,Poulpy21/vexcl