Blender Git Loki

Compositor: Enable suggest-override warning.

Compostior relies heavilly on overridden methods. The override keyword has been added
in this module and is desired. The benefit of the override keyword is
that it reports an error when not applied to a (base) virtual method and report
what will not match when refactoring the code to be closer to blender style guide.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D10846