Fix build warnings when global_motion is off

Change-Id: I69f042e6da5a4b5e4a18853c5f15532dfef0204a
diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 5c8dc4e..5d1e442 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -6084,6 +6084,7 @@
 #endif
   (void)mi_row;
   (void)mi_col;
+  (void)cpi;
 #if CONFIG_GLOBAL_MOTION
   if (this_mode == ZEROMV
 #if CONFIG_EXT_INTER