Fix compile warning

Fixed the compile warning when both global-motion
and warped-motion are disabled.

Change-Id: Ie3ac036fc6c0a15e54a56427452682d7ea7864db

av1/encoder/rdopt.c

@@ -5265,6 +5265,7 @@ static void joint_motion_search(const AV1_COMP *cpi, MACROBLOCK *x,
   int ite, ref;
   struct scale_factors sf;
 #if CONFIG_GLOBAL_MOTION || CONFIG_WARPED_MOTION
+  struct macroblockd_plane *const pd = &xd->plane[0];
   // ic and ir are the 4x4 coordiantes of the sub8x8 at index "block"
   const int ic = block & 1;
   const int ir = (block - ic) >> 1;