qt: Add missing #include <functional>

* lang/qt/src/qgpgmenewcryptoconfig.cpp,
lang/qt/src/threadedjobmixin.h: Include functional.

With GCC 7.0, functional is not included transitively and we get:
In file included from qgpgmedeletejob.h:39:0,

from qgpgmedeletejob.cpp:38:

threadedjobmixin.h:98:33: error: 'function' in namespace 'std'

does not name a template type
 void setFunction(const std::function<T_result()> &function)
                             ^~~~~~~~

std::{function,bind,placeholders,mem_fn} are defined in functional.

References: https://bugzilla.redhat.com/show_bug.cgi?id=1417383

Commit Message amended by Andre Heinecke

• Signed-off-by: Andre Heinecke <aheinecke@intevation.de>
• GnuPG-Bug-Id: T2955