Include what is used

If you use it, you should put an #include in it.

Change-Id: I8919fe2e48adb7cc19173cc44838547a2015288f
diff --git a/runtime/class_reference.h b/runtime/class_reference.h
index 77c296f..7b20612 100644
--- a/runtime/class_reference.h
+++ b/runtime/class_reference.h
@@ -18,6 +18,7 @@
 #define ART_RUNTIME_CLASS_REFERENCE_H_
 
 #include <stdint.h>
+#include <utility>
 
 namespace art {