avconv: set encoder timebase for subtitles.

The actual number (1/1000) will probably require some
discussion/tweaking in the future, but should be good enough for now,
since the timestamps in AVSubtitle are in this timebase by definition.

avconv.c

@@ -2434,6 +2434,7 @@ static int transcode_init(OutputFile *output_files,
 #endif
                 break;
             case AVMEDIA_TYPE_SUBTITLE:
+                codec->time_base = (AVRational){1, 1000};
                 break;
             default:
                 abort();