decoder_api: moved enum decoder_command to decoder_command.h

Minimize header dependencies, again.

src/decoder_api.h

@@ -26,6 +26,7 @@
  *
  */
 
+#include "decoder_command.h"
 #include "decoder_plugin.h"
 #include "input_stream.h"
 #include "replay_gain.h"
@@ -34,13 +35,6 @@
 
 #include <stdbool.h>
 
-enum decoder_command {
-	DECODE_COMMAND_NONE = 0,
-	DECODE_COMMAND_START,
-	DECODE_COMMAND_STOP,
-	DECODE_COMMAND_SEEK
-};
-
 
 /**
  * Notify the player thread that it has finished initialization and