Move the daala_idtx32() function into daala_tx.c.

This commit has no impact on metrics.

Change-Id: I5d3dd031703b48e2efd3aaaebef070569c0c4a2d
diff --git a/av1/common/daala_tx.c b/av1/common/daala_tx.c
index b1519bb..cf96918 100644
--- a/av1/common/daala_tx.c
+++ b/av1/common/daala_tx.c
@@ -4248,6 +4248,11 @@
   for (i = 0; i < 32; i++) output[i] = (tran_low_t)x[i];
 }
 
+void daala_idtx32(const tran_low_t *input, tran_low_t *output) {
+  int i;
+  for (i = 0; i < 32; i++) output[i] = input[i];
+}
+
 #if CONFIG_TX64X64
 void daala_fdct64(const tran_low_t *input, tran_low_t *output) {
   int i;