Add experimental tag for 64x64 transform

Change-Id: I65c04006f6e6eb13ceb22efc1c39915cb3c82b82
diff --git a/configure b/configure
index 15b4092..a95c681 100755
--- a/configure
+++ b/configure
@@ -259,6 +259,7 @@
     ref_mv
     dual_filter
     ext_tx
+    tx64x64
     ext_intra
     filter_intra
     ext_inter