Fix build with gcc 4.8

When trying to build krusader 2.6.0 with gcc 4.8, I got this error
message:
/home/abuild/rpmbuild/BUILD/krusader-2.6.0/krusader/FileSystem/sizecalculator.cpp:
In member function 'void SizeCalculator::slotStatResult(KJob*)':
/home/abuild/rpmbuild/BUILD/krusader-2.6.0/krusader/FileSystem/sizecalculator.cpp:139:98:
error: no matching function for call to
'SizeCalculator::connect(QPointer<KIO::DirectorySizeJob>&, void
(KJob::*)(KJob*, KJob::QPrivateSignal), SizeCalculator* const, void
(SizeCalculator::*)(KJob*))'

connect(m_directorySizeJob, &KIO::Job::result, this,

&SizeCalculator::slotDirectorySizeResult);

                  ^

This patch fixes the problem.

Differential Revision: https://phabricator.kde.org/D5714