avcodec/hevc_mp4toannexb_bsf: silence -Wdiscarded-qualifiers

*poutbuf is non-const, so this casts it explicitly.
This suppresses -Wdiscarded-qualifiers seen in e.g
http://fate.ffmpeg.org/log.cgi?time=20150919100330&log=compile&slot=x86_64-archlinux-gcc-enableshared.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/hevc_mp4toannexb_bsf.c

@@ -134,7 +134,7 @@ static int hevc_mp4toannexb_filter(AVBitStreamFilterContext *bsfc,
                        "The input looks like it is Annex B already\n");
                 ctx->logged_nonmp4_warning = 1;
             }
-            *poutbuf      = buf;
+            *poutbuf      = (uint8_t *)buf;
             *poutbuf_size = buf_size;
             return 0;
         }