Enable ADST for UV channel

derflr +0.202%
hevclf +0.207%
hevcmr +0.095%
hevchr +0.077%

Tested locally on several derf sequences, speed (encoder + decoder)
is slower by less than 1%.

It is part of the EXT_TX experiment, which is to be continued to
explore different transform variants.

Change-Id: I05d44994a62106538a9a241ed8d89bd7c5d14761
diff --git a/configure b/configure
index ac196da..6bb90e3 100755
--- a/configure
+++ b/configure
@@ -264,6 +264,7 @@
     spatial_svc
     fp_mb_stats
     emulate_hardware
+    ext_tx
 "
 CONFIG_LIST="
     dependency_tracking