decoder/List: prefer "mpg123" over "mad"

libmad hasn't been maintained for many many years, while libmpg123 is
still maintained.

Our "mad" plugin can't do streams, but MPD will automatically fall
back to "mad" (or "ffmpeg") for streams.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1954

src/decoder/DecoderList.cxx

@@ -40,12 +40,12 @@
 #include <string.h>
 
 constinit const struct DecoderPlugin *const decoder_plugins[] = {
-#ifdef ENABLE_MAD
-	&mad_decoder_plugin,
-#endif
 #ifdef ENABLE_MPG123
 	&mpg123_decoder_plugin,
 #endif
+#ifdef ENABLE_MAD
+	&mad_decoder_plugin,
+#endif
 #ifdef ENABLE_VORBIS_DECODER
 	&vorbis_decoder_plugin,
 #endif