ART: Fix up small header includes

Test: m
Change-Id: I6978d6eb4b95a6ee810e5a48ca6f5d6c590d4ce1
diff --git a/compiler/optimizing/scheduler_arm.cc b/compiler/optimizing/scheduler_arm.cc
index ea15790..d6eb6e3 100644
--- a/compiler/optimizing/scheduler_arm.cc
+++ b/compiler/optimizing/scheduler_arm.cc
@@ -20,6 +20,7 @@
 #include "code_generator_utils.h"
 #include "common_arm.h"
 #include "mirror/array-inl.h"
+#include "mirror/string.h"
 
 namespace art {
 namespace arm {