8330862: GCBarrierIRExample fails when a different GC is selected via the command line

Reviewed-by: thartmann, dholmes, stefank

test/hotspot/jtreg/testlibrary_tests/ir_framework/examples/GCBarrierIRExample.java

@@ -33,6 +33,7 @@ import java.lang.invoke.MethodHandles;
  * @summary Example test that illustrates the use of the IR test framework for
  *          verification of late-expanded GC barriers.
  * @library /test/lib /
+ * @requires vm.gc.ZGenerational
  * @run driver ir_framework.examples.GCBarrierIRExample
  */