fix: non-virtual-dtor warning

[qwertycxz]

This virtual destructor will resolve the warning of option -Wnon-virtual-dtor in MinGW.

Fixes: #1499

[sylveon]

You are not meant to delete classes which derive from IUnknown. They automatically delete themselves once the ref count reaches 0. Adding the virtual destructor changes the offset of every function in interfaces that derive from IUnknown, breaking the COM ABI.

The correct resolution here is to suppress the warning, not declare the virtual dtor.