kf5backend.cxx: Use 'free()' instead of 'delete'

'strdup' doc [1] mentions:
"The returned pointer must be passed to 'free' to avoid a memory leak."

This silences a "Mismatched free() / delete / delete []" error that
valgrind outputs.

[1] https://en.cppreference.com/w/c/experimental/dynamic/strdup

Change-Id: I28877be762256a7b995c09415a8ad9977b7998c4
Reviewed-on: https://gerrit.libreoffice.org/77974
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

shell/source/backends/kf5be/kf5backend.cxx

@@ -190,7 +190,7 @@ void initQApp(std::map<OUString, css::beans::Optional<css::uno::Any>>& rSettings
     std::unique_ptr<QApplication> app(new QApplication(nFakeArgc, pFakeArgv));
     QObject::connect(app.get(), &QObject::destroyed, app.get(), [nFakeArgc, pFakeArgv]() {
         for (int i = 0; i < nFakeArgc; ++i)
-            delete pFakeArgv[i];
+            free(pFakeArgv[i]);
         delete[] pFakeArgv;
     });