Fix missing import

This compilation error happens if a method in web interface class has a return type.

```
vibe.d/source/vibe/internal/meta/funcattr.d(222): Error: template instance ReturnType!FUNCTION template 'ReturnType' is not defined
vibe.d/source/vibe/web/web.d(609): Error: template instance vibe.internal.meta.funcattr.evaluateOutputModifiers!(recent) error instantiating
```