Clean up mutexes on shutdown.

Change-Id: Ib961f954280c27f8858c4d6fb9010cc833bb28d2
diff --git a/src/intern_table.h b/src/intern_table.h
index 61da28b..0a1554d 100644
--- a/src/intern_table.h
+++ b/src/intern_table.h
@@ -13,6 +13,7 @@
 class InternTable {
  public:
   InternTable();
+  ~InternTable();
   String* Intern(int32_t utf16_length, const char* utf8_data);
   void VisitRoots(Heap::RootVistor* root_visitor, void* arg);