Silence warnings when warped_motion is disabled

Change-Id: I5da893be07b3b9aa4351b5bb1af987f4992370ef
diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 27171e8..272ac57 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -8444,6 +8444,9 @@
   (void)rate_mv;
   (void)is_comp_pred;
   (void)this_mode;
+#if !CONFIG_WARPED_MOTION
+  (void)single_newmv;
+#endif
 
 #if CONFIG_MOTION_VAR || CONFIG_WARPED_MOTION
   MOTION_MODE motion_mode, last_motion_mode_allowed;