Avoid method hiding warning

[dkachuma]

On GPU builds I was getting many warnings of the form

src/coreComponents/constitutiveDrivers/fluid/multiFluid/PVTDriver.hpp(50): warning #997-D: function "geos::ExecutableGroup::execute(geos::real64, geos::real64, geos::integer, geos::integer, geos::real64, geos::DomainPartition &)" is hidden by "geos::PVTDriver::execute" -- virtual function override intended?
    bool execute() override;
         ^

This change makes it more explicit to avoid this warning.