Remove const from int ext_tx_set

The variable was later assigned value in the function.

Change-Id: I93f283a134499a050b46d9dcd6f0c0b4e8d54049
diff --git a/av1/encoder/encodeframe.c b/av1/encoder/encodeframe.c
index 8421237..5207743 100644
--- a/av1/encoder/encodeframe.c
+++ b/av1/encoder/encodeframe.c
@@ -6703,7 +6703,7 @@
   MB_MODE_INFO *mbmi;
   TX_TYPE tx_type, best_tx_nostx;
 #if CONFIG_EXT_TX
-  const int ext_tx_set;
+  int ext_tx_set;
 #endif  // CONFIG_EXT_TX
   int tmp_rate_tx = 0, skip_tx = 0;
   int64_t tmp_dist_tx = 0, rd_tx, bestrd_tx = INT64_MAX;