output: add missing declaration for flb_output_instance_init.
@@ -753,6 +753,7 @@ void flb_output_exit(struct flb_config *config);
void flb_output_set_context(struct flb_output_instance *ins, void *context);
int flb_output_instance_destroy(struct flb_output_instance *ins);
int flb_output_init_all(struct flb_config *config);
+int flb_output_instance_init(struct flb_output_instance *ins, struct flb_config *config);
int flb_output_check(struct flb_config *config);
int flb_output_log_check(struct flb_output_instance *ins, int l);
