__STDC_ISO_10646__ is defined in stdc-predef.h
therefore include it to see if its there on a platform

Signed-off-by: Khem Raj <raj.khem@gmail.com>

Index: libedit-20160903-3.1/src/chartype.h
===================================================================
--- libedit-20160903-3.1.orig/src/chartype.h
+++ libedit-20160903-3.1/src/chartype.h
@@ -29,6 +29,7 @@
 #ifndef _h_chartype_f
 #define _h_chartype_f
 
+#include <stdc-predef.h>
 /* Ideally we should also test the value of the define to see if it
  * supports non-BMP code points without requiring UTF-16, but nothing
  * seems to actually advertise this properly, despite Unicode 3.1 having
