cnn{,_avx2}.c: make some functions static

This quiets some -Wmissing-prototypes warnings.

Bug: aomedia:3416
Change-Id: Ifadbf15b5ee30636f89fe56ee09a40b0f3c3f362
diff --git a/av1/encoder/cnn.c b/av1/encoder/cnn.c
index 598b362..b019ace 100644
--- a/av1/encoder/cnn.c
+++ b/av1/encoder/cnn.c
@@ -138,14 +138,16 @@
   return true;
 }
 
-int check_tensor_equal_dims(TENSOR *t1, TENSOR *t2) {
+#ifndef NDEBUG
+static int check_tensor_equal_dims(TENSOR *t1, TENSOR *t2) {
   return (t1->width == t2->width && t1->height == t2->height);
 }
 
-int check_tensor_equal_size(TENSOR *t1, TENSOR *t2) {
+static int check_tensor_equal_size(TENSOR *t1, TENSOR *t2) {
   return (t1->channels == t2->channels && t1->width == t2->width &&
           t1->height == t2->height);
 }
+#endif  // NDEBUG
 
 void av1_find_cnn_layer_output_size(int in_width, int in_height,
                                     const CNN_LAYER_CONFIG *layer_config,
@@ -189,8 +191,8 @@
   }
 }
 
-void find_cnn_out_channels(const CNN_LAYER_CONFIG *layer_config,
-                           int channels_per_branch[]) {
+static void find_cnn_out_channels(const CNN_LAYER_CONFIG *layer_config,
+                                  int channels_per_branch[]) {
   int branch = layer_config->branch;
   const CNN_BRANCH_CONFIG *branch_config = &layer_config->branch_config;
   for (int b = 0; b < CNN_MAX_BRANCHES; ++b) {
diff --git a/av1/encoder/x86/cnn_avx2.c b/av1/encoder/x86/cnn_avx2.c
index ee93b3d..9c26a56 100644
--- a/av1/encoder/x86/cnn_avx2.c
+++ b/av1/encoder/x86/cnn_avx2.c
@@ -466,7 +466,7 @@
 // As per the layer config set by av1_intra_mode_cnn_partition_cnn_config,
 // the filter_width and filter_height are equal to 2 for layer >= 1. So
 // convolution happens at 2x2 for layer >= 1.
-void cnn_convolve_no_maxpool_padding_valid_2x2_avx2(
+static void cnn_convolve_no_maxpool_padding_valid_2x2_avx2(
     const float **input, int in_width, int in_height, int in_stride,
     const CNN_LAYER_CONFIG *const layer_config, float **output, int out_stride,
     int start_idx, const int cstep, const int channel_step) {