qtquick|examples: Initialize the platform

Fixes a crash.
There's more (unrelated) crashes though, will look at those next.

examples/qtquick/customtitlebar/main.cpp

@@ -40,6 +40,7 @@ int main(int argc, char *argv[])
     QGuiApplication::setAttribute(Qt::AA_UseOpenGLES);
 #endif
     QGuiApplication app(argc, argv);
+    KDDockWidgets::initFrontend(KDDockWidgets::FrontendType::QtQuick);
 
     auto &config = KDDockWidgets::Config::self();
     auto flags = config.flags();