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/vp9_decrypt_test.cc

@@ -47,7 +47,7 @@ TEST(TestDecrypt, DecryptWorksVp9) {
   libvpx_test::IVFVideoSource video("vp90-2-05-resize.ivf");
   video.Init();
 
-  vpx_codec_dec_cfg_t dec_cfg = {0};
+  vpx_codec_dec_cfg_t dec_cfg = vpx_codec_dec_cfg_t();
   VP9Decoder decoder(dec_cfg, 0);
 
   video.Begin();