Fix build with Qt dev branch

/Qt/include/QtCore/qhashfunctions.h:101:35: error: call to function 'qHash' that is neither visible in the template definition nor found by argument-dependent lookup
    Q_DECL_NOEXCEPT_EXPR(noexcept(qHash(t)))

/Qt/include/QtCore/qcompilerdetection.h:1005:43: note: expanded from macro 'Q_DECL_NOEXCEPT_EXPR'

/Qt/include/QtCore/qhash.h:873:13: note: in instantiation of exception specification for 'qHash<Akonadi::Collection>' requested here
        h = qHash(akey, d->seed);

/Qt/include/QtCore/qhash.h:698:19: note: in instantiation of member function 'QHash<Akonadi::Collection, QList<Akonadi::Item> >::findNode' requested here
    Node **node = findNode(akey, &h);

/kf5/kdepimlibs/akonadi/src/core/jobs/trashrestorejob.cpp:126:53: note: in instantiation of member function 'QHash<Akonadi::Collection, QList<Akonadi::Item> >::operator[]' requested here
        const Item::List &items = restoreCollections[Collection(var.toInt())];

/kf5/kdepimlibs/akonadi/src/core/collection.h:377:25: note: 'qHash' should be declared prior to the call site or in namespace 'Akonadi'