Add a missing SHARED_LOCKS_REQUIRED(mutator_lock_).

This was caught by Clang 3.6 using -Wthread-safety-analysis.

Change-Id: If2f25331f111ba6c67570e5aece1fba38f714d05
diff --git a/runtime/jni_internal.cc b/runtime/jni_internal.cc
index 1dcfcab..4797e69 100644
--- a/runtime/jni_internal.cc
+++ b/runtime/jni_internal.cc
@@ -566,7 +566,8 @@
     return soa.AddLocalReference<jobject>(decoded_obj);
   }
 
-  static void DeleteLocalRef(JNIEnv* env, jobject obj) {
+  static void DeleteLocalRef(JNIEnv* env, jobject obj)
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
     if (obj == nullptr) {
       return;
     }