AudioFileFlac: allow compilation withoug FLAC support

Compilation of AudioFileFlac source files failed when there's been
no FLAC support on the system. Fix this by adding according #ifdef's.

src/core/audio/AudioFileFlac.cpp

@@ -25,6 +25,8 @@
 
 #include "AudioFileFlac.h"
 
+#ifdef LMMS_HAVE_FLAC
+
 #include <QtDebug>
 
 #include "lmms_basics.h"
@@ -122,4 +124,6 @@ AudioFileFlac::~AudioFileFlac()
 	finishEncoding();
 }
 
+#endif
+
 /* vim: set tw=0 noexpandtab: */