moved global variable "ob" to outputBuffer.h

This releases several include file dependencies.  As a side effect,
"CHUNK_SIZE" isn't defined by decoder_api.h anymore, so we have to
define it directly in the plugins which need it.  It just isn't worth
it to add it to the decoder plugin API.

src/decoder_api.c

@@ -25,6 +25,7 @@
 #include "utils.h"
 #include "normalize.h"
 #include "playerData.h"
+#include "outputBuffer.h"
 #include "gcc.h"
 
 void decoder_plugin_register(struct decoder_plugin *plugin)