Fix compiler issue when combining gizmo-mfv and COLIBRE cooling

A line in the COLIBRE cooling code was directly accessing p->mass rather than using hydro_get_mass(p), which does not work for a scheme like gizmo-mfv that does not have a particle mass. This caused a compilation error, which is now fixed.

Should we be worried about the fact that the cooling code in the COLIBRE fork does things completely differently and does not have this issue?

@jbraspenning Could you let me know if there are any other unexpected issues with the cooling=COLIBRE hydro=gizmo-mfv combination?