schedule_transforms.h: directly include schedule_tree_elem.h

Currently, this file is included transitively by schedule_tree.h.  An
upcoming commit will swap the include order, so this file needs to
include schedule_tree_elem.h directly.

Author: ftynse

tc/core/polyhedral/schedule_transforms.h

@@ -25,6 +25,7 @@
 #include "tc/core/polyhedral/mapping_types.h"
 #include "tc/core/polyhedral/options.h"
 #include "tc/core/polyhedral/schedule_tree.h"
+#include "tc/core/polyhedral/schedule_tree_elem.h"
 #include "tc/external/isl.h"
 
 namespace tc {