Call QCoreApplication::exit instead of stdlib exit to terminate if platform fails

Summary:
We need to properly tear down the application - this can be achieved
through QCoreApplication::exit. Otherwise there is a chance that the
cleanup handling crashes.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2136

main_wayland.cpp

@@ -135,7 +135,7 @@ void ApplicationWayland::createBackend()
     connect(platform(), &Platform::initFailed, this,
         [] () {
             std::cerr <<  "FATAL ERROR: backend failed to initialize, exiting now" << std::endl;
-            ::exit(1);
+            QCoreApplication::exit(1);
         }
     );
     platform()->init();