decoder/{vorbis,mpcdec}: use `using` instead of `typedef`

src/decoder/plugins/MpcdecDecoderPlugin.cxx

@@ -45,7 +45,7 @@ struct mpc_decoder_data {
 static constexpr Domain mpcdec_domain("mpcdec");
 
 static constexpr SampleFormat mpcdec_sample_format = SampleFormat::S24_P32;
-typedef SampleTraits<mpcdec_sample_format> MpcdecSampleTraits;
+using MpcdecSampleTraits = SampleTraits<mpcdec_sample_format>;
 
 static mpc_int32_t
 mpc_read_cb(mpc_reader *reader, void *ptr, mpc_int32_t size)

src/decoder/plugins/VorbisDecoderPlugin.cxx

@@ -50,8 +50,8 @@ class VorbisDecoder final : public OggDecoder {
 	typedef int16_t out_sample_t;
 #else
 	static constexpr SampleFormat sample_format = SampleFormat::FLOAT;
-	typedef float in_sample_t;
-	typedef float out_sample_t;
+	using in_sample_t = float;
+	using out_sample_t = float;
 #endif
 
 	unsigned remaining_header_packets;