build: enable IWYU on mac

src/nvim/ex_docmd.c

@@ -9,6 +9,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/types.h>
 
 #include "auto/config.h"
 #include "nvim/arglist.h"