--disable-mpg123 needs the header conditionally included, too

src/modules/sound/lullaby/Sound.cpp

@@ -23,11 +23,14 @@
 #include "Sound.h"
 
 #include "ModPlugDecoder.h"
-#include "Mpg123Decoder.h"
 #include "VorbisDecoder.h"
 #include "GmeDecoder.h"
 //#include "FLACDecoder.h"
 
+#ifndef LOVE_NOMPG123
+#	include "Mpg123Decoder.h"
+#endif // LOVE_NOMPG123
+
 namespace love
 {
 namespace sound