audio_format: add DSD sample format

Basic support for Direct Stream Digital.  No conversion yet, and no
decoder/output plugin support.

src/decoder/flac_pcm.c

@@ -103,6 +103,8 @@ flac_convert(void *dest,
 
 	case SAMPLE_FORMAT_S24:
 	case SAMPLE_FORMAT_FLOAT:
+	case SAMPLE_FORMAT_DSD:
+	case SAMPLE_FORMAT_DSD_LSBFIRST:
 	case SAMPLE_FORMAT_UNDEFINED:
 		/* unreachable */
 		assert(false);