StringReference comparator operator is missing const

Test: build
Bug: 37752547
Change-Id: I0557595f92573c782ee7f6420931f792d2df63d9
diff --git a/runtime/string_reference.h b/runtime/string_reference.h
index 0fc06e6..6ba4773 100644
--- a/runtime/string_reference.h
+++ b/runtime/string_reference.h
@@ -41,7 +41,7 @@
 
 // Compare only the reference and not the string contents.
 struct StringReferenceComparator {
-  bool operator()(const StringReference& a, const StringReference& b) {
+  bool operator()(const StringReference& a, const StringReference& b) const {
     if (a.dex_file != b.dex_file) {
       return a.dex_file < b.dex_file;
     }