Call .rs.dtor() when tearing down Scripts.

BUG=5186750

This allows us to properly reference count any globals (static or extern) that
need to potentially be cleaned up.

Change-Id: I03d2c38c1e7a4ca96c40003d2eeecb6f395d5835
diff --git a/rsScript.h b/rsScript.h
index c0324dd..d645421 100644
--- a/rsScript.h
+++ b/rsScript.h
@@ -73,6 +73,8 @@
     void setVar(uint32_t slot, const void *val, size_t len);
     void setVarObj(uint32_t slot, ObjectBase *val);
 
+    virtual bool freeChildren();
+
     virtual void runForEach(Context *rsc,
                             const Allocation * ain,
                             Allocation * aout,