ls: include cleanup

Don't include headers which are not used.  Fix some includes in
decoder_thread.c.

src/decoder_thread.c

@@ -20,6 +20,7 @@
 #include "decoder_thread.h"
 #include "decoder_control.h"
 #include "decoder_internal.h"
+#include "decoder_list.h"
 #include "player_control.h"
 #include "pipe.h"
 #include "song.h"
@@ -30,6 +31,8 @@
 
 #include <glib.h>
 
+#include <unistd.h>
+
 static bool
 decoder_stream_decode(const struct decoder_plugin *plugin,
 		      struct decoder *decoder,