Add inclusion of functional header.
Add inclusion of functional header.

d-tk/dtk,d-tk/dtk,d-tk/dtk,rdebroiz/dtk,rdebroiz/dtk,NicolasSchnitzler/dtk,NicolasSchnitzler/dtk,d-tk/dtk,d-tk/dtk,rdebroiz/dtk,NicolasSchnitzler/dtk,rdebroiz/dtk,NicolasSchnitzler/dtk,d-tk/dtk