cnn: add missing braces

Fixes build for GCC 4.

Change-Id: I187b29a7b03efbb93df1c582d3fcc01d32c7d503
diff --git a/av1/encoder/cnn.c b/av1/encoder/cnn.c
index 29fc87d..6b2a55a 100644
--- a/av1/encoder/cnn.c
+++ b/av1/encoder/cnn.c
@@ -832,8 +832,8 @@
                        int in_stride, const CNN_CONFIG *cnn_config,
                        const CNN_THREAD_DATA *thread_data,
                        CNN_MULTI_OUT *output_struct) {
-  TENSOR tensor1[CNN_MAX_BRANCHES] = { 0 };
-  TENSOR tensor2[CNN_MAX_BRANCHES] = { 0 };
+  TENSOR tensor1[CNN_MAX_BRANCHES] = { { 0 } };
+  TENSOR tensor2[CNN_MAX_BRANCHES] = { { 0 } };
 
   float **output[CNN_MAX_BRANCHES];
   const int *out_chs = output_struct->output_channels;