tests: use vpx_codec_dec_cfg_t() to initialize vars

0-initializes [1] and removes a warning for missing initializer fields

[1] http://en.cppreference.com/w/cpp/language/value_initialization

Change-Id: I364248010c8fa663c71d8f06a9999c730e92db4c

test/user_priv_test.cc

@@ -47,7 +47,7 @@ string DecodeFile(const string &filename) {
   libvpx_test::WebMVideoSource video(filename);
   video.Init();
 
-  vpx_codec_dec_cfg_t cfg = {0};
+  vpx_codec_dec_cfg_t cfg = vpx_codec_dec_cfg_t();
   libvpx_test::VP9Decoder decoder(cfg, 0);
 
   libvpx_test::MD5 md5;