Import: Disable thread import for OBJ meshes

This can currently lead to deadlocks, possibly due to some race condition
in the Vulkan renderer.

Works around #48265.

editor/import/resource_importer_obj.h

@@ -64,6 +64,9 @@ public:
 
 	virtual Error import(const String &p_source_file, const String &p_save_path, const Map<StringName, Variant> &p_options, List<String> *r_platform_variants, List<String> *r_gen_files = nullptr, Variant *r_metadata = nullptr) override;
 
+	// Threaded import can currently cause deadlocks, see GH-48265.
+	virtual bool can_import_threaded() const override { return false; }
+
 	ResourceImporterOBJ();
 };