Add opt_ref_mv experimental flag

Change-Id: I24cb257367775608b2b6e4d45d07c30b511af251
diff --git a/configure b/configure
index b12f279..8b65a32 100755
--- a/configure
+++ b/configure
@@ -350,6 +350,7 @@
     obu
     amvr
     lpf_sb
+    opt_ref_mv
 "
 CONFIG_LIST="
     dependency_tracking