[wayland] Reset QT_QPA_PLATFORM to wayland after starting Application

We only need to force QT_QPA_PLATFORM to be our own plugin when starting
the application. As KWin starts other processes (e.g. kcmshell5) we need
to ensure that the env variable has a sane value. No other process can
load our QPA plugin after all.

Reviewed-By: Bhushan Shah

main_wayland.cpp

@@ -380,6 +380,8 @@ int main(int argc, char * argv[])
 #endif
     KWin::ApplicationWayland a(argc, argv);
     a.setupTranslator();
+    // reset QT_QPA_PLATFORM to a sane value for any processes started from KWin
+    setenv("QT_QPA_PLATFORM", "wayland", true);
 
     KWin::Application::createAboutData();