add linux default

example/linux/main.cc

@@ -0,0 +1,10 @@
+#include "my_application.h"
+
+int main(int argc, char** argv) {
+  // Only X11 is currently supported.
+  // Wayland support is being developed: https://github.com/flutter/flutter/issues/57932.
+  gdk_set_allowed_backends("x11");
+
+  g_autoptr(MyApplication) app = my_application_new();
+  return g_application_run(G_APPLICATION(app), argc, argv);
+}