GeneratorObjects: Fix compilation with c++20.
With c++20, GenParticle*==HepMcParticleLink was ambiguous.
- The second argument could convert to GenParticle* and then compare using the built-in == operation.
- The first argument could convert to HepMcParticleLink and the compare using HepMcParticleLink::operator==.
The second of these is only allowed in c++20 due to the fact that operator== is implicitly reversable (otherwise, we wouldn't be able to convert the first argument as this is a member function).
Add an explicit operator== for this case to disambiguate (choosing the first alternative).