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.c

@@ -19,6 +19,7 @@
 
 #include "decoder_internal.h"
 #include "decoder_list.h"
+#include "decode.h"
 #include "audio.h"
 
 #include "utils.h"