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>

MAINTAINERS

@@ -132,6 +132,7 @@ Generic Parts:
     tableprint.c, tableprint.h          Reimar Doeffinger
   fixed point FFT:
     fft*                                Zeljko Lukac
+  Text Subtitles                        Clément Bœsch
 
 Codecs:
   4xm.c                                 Michael Niedermayer