moved variable "dc" to decode.h

Now that "dc" is available here, we don't have to pass it to
decoder_is_idle() and decoder_is_starting() anymore.

src/decoder_control.c

@@ -17,7 +17,8 @@
  */
 
 #include "decode.h"
-#include "playerData.h"
+
+struct decoder_control dc;
 
 void dc_command_wait(Notify *notify)
 {