Fix build if QtQml was built with -no-qml-debug

I cross-compiling for the recently released Arm Morello boards and the
QtQml debugger is not yet compatible with CHERI-enabled architectures so
I had to build QtQml with -no-qml-debug.

shell/main.cpp

@@ -56,9 +56,11 @@ void filterConnectionSyntaxWarning(QLoggingCategory *category)
 
 int main(int argc, char *argv[])
 {
+#if QT_CONFIG(qml_debug)
     if (qEnvironmentVariableIsSet("PLASMA_ENABLE_QML_DEBUG")) {
         QQmlDebuggingEnabler debugger;
     }
+#endif
     // Plasma scales itself to font DPI
     // on X, where we don't have compositor scaling, this generally works fine.
     // also there are bugs on older Qt, especially when it comes to fractional scaling