Bug#838829: Try to add Generics declaration to class definition

[Daniel Espinosa]

As for bug:

https://bugzilla.gnome.org/show_bug.cgi?id=770449

Valac have started to check types in other places and if GitHub manual is
used to check generic classes declaration, as for:

https://github.com/valadate-project/the-vala-manual/blob/master/en/generics-examples.md

then your example should be:

public abstract class basecl<T> : Object {
  public abstract void test<T>(T parameter);
}

public class inheritcl<T> : basecl {
  public override void test<T>(T parameter) {
    stdout.printf("Just a test!\n");
  }
}

May you want to test this code and change this bug, as this may it is not,
but a code update required due to recent additional checks in valac.