SolarMutexGuard needed here

assert seen in dbgutil

Change-Id: I09cab9e4fa4ce81d71518d87c4a571a467ead52d
Reviewed-on: https://gerrit.libreoffice.org/66327
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

vcl/unx/gtk3/gtk3gtkinst.cxx

@@ -3018,6 +3018,7 @@ private:
 
     static gboolean launch_overflow_switch_page(GtkInstanceNotebook* pThis)
     {
+        SolarMutexGuard aGuard;
         pThis->signal_overflow_switch_page();
         return false;
     }