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_api.h

@@ -30,6 +30,7 @@
 #include "replayGain.h"
 #include "tag.h"
 #include "playerData.h"
+#include "decode.h"
 
 
 /* valid values for streamTypes in the InputPlugin struct: */