fix declaration

src/datawizard/interfaces/data_interface.h

@@ -57,7 +57,7 @@ void _starpu_data_free_interfaces(starpu_data_handle_t handle)
 extern
 int _starpu_data_handle_init(starpu_data_handle_t handle, struct starpu_data_interface_ops *interface_ops, unsigned int mf_node);
 
-struct starpu_arbiter *_starpu_global_arbiter;
+extern struct starpu_arbiter *_starpu_global_arbiter;
 extern void _starpu_data_interface_init(void) STARPU_ATTRIBUTE_INTERNAL;
 extern int _starpu_data_check_not_busy(starpu_data_handle_t handle) STARPU_ATTRIBUTE_INTERNAL;
 extern void _starpu_data_interface_shutdown(void) STARPU_ATTRIBUTE_INTERNAL;