Fix warning in core_class_hierarchy.hpp

[praetorian20]

core_class_hierarchy.hpp contains classes with the following
inheritance hierarchy

derived_private_t -> base_t
multi_derived_t   -> derived_private_t, base_t

Since multi_derived_t contains base_t as a direct base and as a base
via derived_private_t, this creates warnings about base_t being
inaccessible because it's ambiguous which base_t instance is being
referred to.

Changed the inheritance hierarchy to virtual inheritance so that
derived_private_t and multi_derived_t inherit base_t virtually. Now
there's only a single instance of base_t in multi_derived_t and the
warning is avoided.

Fixes #46

Change-Id: I3a52a666460bea2cc3c66b1855288d757f93f507

[praetorian20]

Just ignore this for a day. CI is failing because some fixes for CastXML from #35 are missing. Once you merge that, I'll rebase this and it'll pass.

[iMichka]

It's ok; the two things are unrelated and this patch was good to go. Further builds on the develop branch should be green once #35 is done. Do not forget to squash/rebase #35 so I can merge it too.

Thanks for the fix !

[praetorian20]

I've already squashed #35, forgot to comment that I did that.