renamed InputPlugin to struct decoder_plugin

"decoder plugin" is a better name than "input plugin", since the
plugin does not actually do the input - InputStream does.  Also don't
use typedef, so we can forward-declare it if required.

src/decoder_internal.h

@@ -23,7 +23,7 @@
 #include "pcm_utils.h"
 
 struct decoder {
-	InputPlugin *plugin;
+	struct decoder_plugin *plugin;
 
 	ConvState conv_state;
 };