Clean up mutexes on shutdown.

Change-Id: Ib961f954280c27f8858c4d6fb9010cc833bb28d2
diff --git a/src/jni_internal.h b/src/jni_internal.h
index 1a4235d..a6fce05 100644
--- a/src/jni_internal.h
+++ b/src/jni_internal.h
@@ -21,6 +21,7 @@
 
 struct JavaVMExt {
   JavaVMExt(Runtime* runtime, bool check_jni, bool verbose_jni);
+  ~JavaVMExt();
 
   /*
    * Load native code from the specified absolute pathname.  Per the spec,