Fix build with isl >= 0.20.

Backport from https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86724

--- gcc-6.4.0/gcc/graphite.h.old	2016-01-25 18:19:20.909140000 -0600
+++ gcc-6.4.0/gcc/graphite.h	2022-06-21 02:51:53.660000000 -0500
@@ -40,6 +40,8 @@
 #ifdef HAVE_ISL_OPTIONS_SET_SCHEDULE_SERIALIZE_SCCS
 /* isl 0.15 or later.  */
 #include <isl/schedule_node.h>
+#include <isl/id.h>
+#include <isl/space.h>
 
 #else
 /* isl 0.14 or 0.13.  */