fix: solve leak sanitizer on graceful shutdown

I am testing by simply calling `pipeline_cancel` on the first pipeline
execution.

include/pipeline.h

@@ -38,8 +38,10 @@ typedef struct {
 
 // create a new pipeline_conf struct instance based on a configuration line.
 optional_pipeline_conf pipeline_create(const char* config_line);
+void pipeline_event_destroy(pipeline_event* ev);
 void pipeline_destroy(pipeline_conf* conf);
 void pipeline_register(pthread_t thread);
 void pipeline_loop();
+void pipeline_cancel();
 
 #endif