deskutils/fet: Fix build on 11.1

In file included from engine/timetableexport.cpp:46:
engine/timetable_defs.h:38:20: error: unknown type name 'constexpr'
template <class T> constexpr std::add_const_t<T>& qAsConst(T& t) noexcept

^

http://beefy10.nyi.freebsd.org/data/111i386-default/478099/logs/fet-5.37.1.log

PR: 230523
Approved by: maintainer