Dict type alias fwd decl: underlying type not fwd declared

[Axel-Naumann]

Reported by ALICE:

G__O2DetectorsVertexing dictionary forward declarations' payload:16:53: error: no template named 'DCAFitterN'
namespace o2{namespace vertexing{using DCAFitter2 = DCAFitterN<2, o2::track::TrackParCov>;}}
                                                    ^
G__O2DetectorsVertexing dictionary forward declarations' payload:17:53: error: no template named 'DCAFitterN'
namespace o2{namespace vertexing{using DCAFitter3 = DCAFitterN<3, o2::track::TrackParCov>;}}
                                                    ^

[Axel-Naumann]

Test: root-project/roottest#723