Add dual_filter into the experimental flag list

Change-Id: I3f240fb46716b70e7c0ddd40660f55e1285875cd
diff --git a/configure b/configure
index eedbf7f..4fd5d29 100755
--- a/configure
+++ b/configure
@@ -273,6 +273,7 @@
     emulate_hardware
     var_tx
     ref_mv
+    dual_filter
     ext_tx
     misc_fixes
     ext_intra