av1_txfm_test.h: move template function to .cc

Quiets a `-Wunused-template` warning. This matches the other templates
in the header: `fliplr`, `flipud`, `fliplrud`.

Change-Id: I24d65c90730bd44b22bc7e96900b497bb560adbb
(cherry picked from commit c94d9ed9b9705394b31ba9ec98ad34086c8bd370)

test/av1_txfm_test.cc

diff --git a/test/av1_txfm_test.cc b/test/av1_txfm_test.cc
index 5d633d3..55d33a6 100644
--- a/test/av1_txfm_test.cc
+++ b/test/av1_txfm_test.cc
@@ -342,6 +342,23 @@
   }
 }
 
+template <typename Type1, typename Type2>
+double compute_avg_abs_error(const Type1 *a, const Type2 *b, const int size) {
+  double error = 0;
+  for (int i = 0; i < size; i++) {
+    error += fabs(static_cast<double>(a[i]) - static_cast<double>(b[i]));
+  }
+  error = error / size;
+  return error;
+}
+
+template double compute_avg_abs_error<int32_t, double>(const int32_t *a,
+                                                       const double *b,
+                                                       const int size);
+template double compute_avg_abs_error<uint16_t, uint16_t>(const uint16_t *a,
+                                                          const uint16_t *b,
+                                                          const int size);
+
 template <typename Type>
 void fliplr(Type *dest, int width, int height, int stride) {
   for (int r = 0; r < height; ++r) {

test/av1_txfm_test.h

diff --git a/test/av1_txfm_test.h b/test/av1_txfm_test.h
index e50dace..8328f0d 100644
--- a/test/av1_txfm_test.h
+++ b/test/av1_txfm_test.h
@@ -57,15 +57,7 @@
 void reference_hybrid_2d(double *in, double *out, TX_TYPE tx_type,
                          TX_SIZE tx_size);
 template <typename Type1, typename Type2>
-static double compute_avg_abs_error(const Type1 *a, const Type2 *b,
-                                    const int size) {
-  double error = 0;
-  for (int i = 0; i < size; i++) {
-    error += fabs(static_cast<double>(a[i]) - static_cast<double>(b[i]));
-  }
-  error = error / size;
-  return error;
-}
+double compute_avg_abs_error(const Type1 *a, const Type2 *b, const int size);
 
 template <typename Type>
 void fliplr(Type *dest, int width, int height, int stride);