config2help: add missing ctype.h include

This file uses isspace which is defined in the ctype.h header.
diff --git a/scripts/config2help.c b/scripts/config2help.c
index 6b32fc4..6ec5e84 100644
--- a/scripts/config2help.c
+++ b/scripts/config2help.c
@@ -1,5 +1,6 @@
 //#include "toys.h"
 
+#include <ctype.h>
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>