build: enable IWYU on mac

src/nvim/marktree.c

@@ -47,6 +47,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/types.h>
 
 #include "klib/kvec.h"
 #include "nvim/garray.h"