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/libs/rs/rs_hal.h b/libs/rs/rs_hal.h
index 21dff218..b8d7351 100644
--- a/libs/rs/rs_hal.h
+++ b/libs/rs/rs_hal.h
@@ -90,6 +90,7 @@
                               uint32_t usrLen,
                               const RsScriptCall *sc);
         void (*invokeInit)(const Context *rsc, Script *s);
+        void (*invokeFreeChildren)(const Context *rsc, Script *s);
 
         void (*setGlobalVar)(const Context *rsc, const Script *s,
                              uint32_t slot,