Ensure ext-tx is enabled when mrc-tx is enabled

When ext-tx is disabled, the mrc-tx implementation is not complete,
so they must be enabled together for now.

Change-Id: Ib049f0e15023272c44a905581842db0626cdf14d

configure

diff --git a/configure b/configure
index 07d3a8d..b3f713e 100755
--- a/configure
+++ b/configure
@@ -541,6 +541,7 @@
     enabled pvq && disable_feature highbitdepth
     enabled pvq && disable_feature lgt
     enabled pvq && disable_feature mrc_tx
+    enabled mrc_tx && enable_feature ext_tx
     enabled palette_throughput && soft_enable palette
     enabled ext_delta_q && soft_enable delta_q
     enabled txk_sel && soft_enable lv_map