Fix override signature problem.

ddox doesn't build with the current github master of dmd. This fixes one of the problems (a function which overrides a base class function is missing the `const` that the base class function has). There's a whole pile of errors beyond this, but I think that they're errors in vibe-d, because they seem to be in `./.dub/packages/vibe-d/source/vibe/templ/diet.d`. So, this isn't enough to get ddox building with the latest git master of dmd (and this fix at least looks like it's a real bug rather than a regression in dmd), but it does fix one problem with it.