patch 9.0.1336: functions without arguments are not always declared properly
Problem: Functions without arguments are not always declared properly.
Solution: Use "(void)" instead of "()". (Yegappan Lakshmanan, closes #12031)
diff --git a/src/vim.h b/src/vim.h
index db38da2..d1ec26c 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -2287,6 +2287,9 @@
/*
* Type for the callback function that is invoked after an option value is
* changed to validate and apply the new value.
+ *
+ * Returns NULL if the option value is valid is successfully applied.
+ * Otherwise returns an error message.
*/
typedef char *(*opt_did_set_cb_T)(optset_T *args);