vp10: fix compiler warning with --enable-universal_hp.

Change-Id: I0d7ca20bdd0fc868b28b0755e3114a4499056f45

vp10/common/entropymv.c

@@ -129,6 +129,7 @@ MV_CLASS_TYPE vp10_get_mv_class(int z, int *offset) {
 
 int vp10_use_mv_hp(const MV *ref) {
 #if CONFIG_UNIVERSAL_HP
+  (void) ref;
   return 1;
 #else
   return (abs(ref->row) >> 3) < COMPANDED_MVREF_THRESH &&