[wallpapers/image] Seed random number generator

Summary:
Otherwise when we randomise the wallpaper order we will always get the
same results.

CCBUG: 413463

Test Plan: qDebug + ran twice

Reviewers: #plasma, ngraham, davidre

Reviewed By: ngraham, davidre

Subscribers: davidre, apol, anthonyfieroni, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24985

wallpapers/image/slidefiltermodel.cpp

@@ -31,6 +31,7 @@ SlideFilterModel::SlideFilterModel(QObject* parent)
     , m_SortingMode{Image::Random}
     , m_usedInConfig{false}
 {
+    srand(time(nullptr));
     setSortCaseSensitivity(Qt::CaseSensitivity::CaseInsensitive);
     connect(this, &SlideFilterModel::usedInConfigChanged, this, &SlideFilterModel::invalidateFilter);
 }