MAINTAINERS: Add ubitux for text subtitles

ubitux maintains them. It seems it was forgotten to update MAINTAINERs though

See: Re: [FFmpeg-devel] [PATCH] MAINTAINERS: add Subtitle maintainer
     Message-ID: <20140713120546.GC26262@leki>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>