calligra/3.0 not building with stable branch of Applications

[Jonathan Riddell]

Calligra's calligras/3.0 branch current doesn't build (with
Applications/17.12 builds)

https://build.neon.kde.org/job/xenial_stable_calligra_calligra_bin_amd64/105/console

14:10:54 /workspace/build/plan/plugins/filters/icalendar/export/icalendarexport.cpp:
In member function ‘void
ICalendarExport::createTodos(KCalCore::Calendar::Ptr, const
KPlato::Node*, long int, KCalCore::Todo::Ptr)’:
14:10:54 /workspace/build/plan/plugins/filters/icalendar/export/icalendarexport.cpp:144:41:
error: ‘KDateTime’ was not declared in this scope
14:10:54          todo->setDtStart( KDateTime( st ) );
14:10:54                                          ^