Move the daala_idtx8() function into daala_tx.c.

This commit has no impact on metrics.

Change-Id: I706b60232d45910b91f17544af20eaf8581b5ab5
diff --git a/av1/common/daala_tx.c b/av1/common/daala_tx.c
index 5aeae16..1b62771 100644
--- a/av1/common/daala_tx.c
+++ b/av1/common/daala_tx.c
@@ -4184,6 +4184,11 @@
   for (i = 0; i < 8; i++) output[i] = (tran_low_t)x[i];
 }
 
+void daala_idtx8(const tran_low_t *input, tran_low_t *output) {
+  int i;
+  for (i = 0; i < 8; i++) output[i] = input[i];
+}
+
 void daala_fdct16(const tran_low_t *input, tran_low_t *output) {
   int i;
   od_coeff x[16];