Cast outputHandler to the proper type in helpers.c.

helpers.c

diff --git a/helpers.c b/helpers.c
index 56e4442..733b618 100644
--- a/helpers.c
+++ b/helpers.c
@@ -14,5 +14,5 @@
 
 void set_output_handler(yaml_emitter_t *e)
 {
-    yaml_emitter_set_output(e, outputHandler, (void *)e);
+    yaml_emitter_set_output(e, (yaml_write_handler_t*)outputHandler, (void *)e);
 }