LibCore: Make Core::s_main_event_loop actually global

This was accidentally per-TU, as it was declared "static" in the header.

Userland/Libraries/LibCore/EventLoop.cpp

@@ -62,7 +62,7 @@ struct EventLoop::Private {
 };
 
 // The main event loop is global to the program, so it may be accessed from multiple threads.
-// NOTE: s_main_event_loop is not declared here as it is needed in the header.
+Threading::MutexProtected<EventLoop*> s_main_event_loop;
 static Threading::MutexProtected<NeverDestroyed<IDAllocator>> s_id_allocator;
 static Threading::MutexProtected<RefPtr<InspectorServerConnection>> s_inspector_server_connection;