Fix compilation errors when setting kTracing = true.

Change-Id: I946c98021166bd099983a8f50a18b57d8eac6af6
Signed-off-by: Douglas Leung <douglas@mips.com>
diff --git a/runtime/interpreter/interpreter_common.h b/runtime/interpreter/interpreter_common.h
index 88a8468..d785aff 100644
--- a/runtime/interpreter/interpreter_common.h
+++ b/runtime/interpreter/interpreter_common.h
@@ -38,6 +38,7 @@
 #include "mirror/class-inl.h"
 #include "mirror/object-inl.h"
 #include "mirror/object_array-inl.h"
+#include "mirror/string-inl.h"
 #include "object_utils.h"
 #include "ScopedLocalRef.h"
 #include "scoped_thread_state_change.h"