Fix build failure in scheduler_arm with ART_USE_OLD_ARM_BACKEND=1

Test: m ART_USE_VIXL_ARM_BACKEND=true test-art-host
Test: m ART_USE_VIXL_ARM_BACKEND=true test-art-target

Change-Id: Id6019ec26a9e9ffe5c0ca7eb2535f094ea009763
diff --git a/compiler/optimizing/scheduler_arm.h b/compiler/optimizing/scheduler_arm.h
index 8d5e4f3..cb679fc 100644
--- a/compiler/optimizing/scheduler_arm.h
+++ b/compiler/optimizing/scheduler_arm.h
@@ -17,7 +17,11 @@
 #ifndef ART_COMPILER_OPTIMIZING_SCHEDULER_ARM_H_
 #define ART_COMPILER_OPTIMIZING_SCHEDULER_ARM_H_
 
+#ifdef ART_USE_OLD_ARM_BACKEND
+#include "code_generator_arm.h"
+#else
 #include "code_generator_arm_vixl.h"
+#endif
 #include "scheduler.h"
 
 namespace art {