encoder/vorbis: generate end-of-stream packet when playback ends

Add the encoder_plugin method end().  This is important for the
recorder plugin.

src/encoder/flac_encoder.c

@@ -354,6 +354,7 @@ const struct encoder_plugin flac_encoder_plugin = {
 	.finish = flac_encoder_finish,
 	.open = flac_encoder_open,
 	.close = flac_encoder_close,
+	.end = flac_encoder_flush,
 	.flush = flac_encoder_flush,
 	.write = flac_encoder_write,
 	.read = flac_encoder_read,

src/encoder/twolame_encoder.c

@@ -300,6 +300,7 @@ const struct encoder_plugin twolame_encoder_plugin = {
 	.finish = twolame_encoder_finish,
 	.open = twolame_encoder_open,
 	.close = twolame_encoder_close,
+	.end = twolame_encoder_flush,
 	.flush = twolame_encoder_flush,
 	.write = twolame_encoder_write,
 	.read = twolame_encoder_read,

src/encoder/vorbis_encoder.c

@@ -405,6 +405,7 @@ const struct encoder_plugin vorbis_encoder_plugin = {
 	.finish = vorbis_encoder_finish,
 	.open = vorbis_encoder_open,
 	.close = vorbis_encoder_close,
+	.end = vorbis_encoder_pre_tag,
 	.flush = vorbis_encoder_flush,
 	.pre_tag = vorbis_encoder_pre_tag,
 	.tag = vorbis_encoder_tag,