decoder/opus: move everything into anonymous namespace

src/decoder/plugins/OpusDecoderPlugin.cxx

@@ -40,6 +40,8 @@
 
 #include <string.h>
 
+namespace {
+
 static constexpr opus_int32 opus_sample_rate = 48000;
 
 /**
@@ -511,6 +513,8 @@ static const char *const opus_mime_types[] = {
 	nullptr
 };
 
+} /* anonymous namespace */
+
 const struct DecoderPlugin opus_decoder_plugin = {
 	"opus",
 	mpd_opus_init,