Add maintainer for WebVTT demuxer and muxer

Matthew J Heaney is added to the list of ffmpeg maintainers, for the
WebVTT demuxer and muxer.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

MAINTAINERS

@@ -407,6 +407,7 @@ Muxers/Demuxers:
   voc.c                                 Aurelien Jacobs
   wav.c                                 Michael Niedermayer
   wc3movie.c                            Mike Melanson
+  webvtt*                               Matthew J Heaney
   westwood.c                            Mike Melanson
   wtv.c                                 Peter Ross
   wv.c                                  Kostya Shishkov