flac, mpc, ogg, wavpack: include unistd.h for SEEK_SET

SEEK_SET is defined by unistd.h.  Explicitly include it.

src/decoder/oggflac_plugin.c

@@ -23,6 +23,7 @@
 #include "_ogg_common.h"
 
 #include <OggFLAC/seekable_stream_decoder.h>
+#include <unistd.h>
 
 static void oggflac_cleanup(FlacData * data,
 			    OggFLAC__SeekableStreamDecoder * decoder)