ffmpeg_opt: Constify hwaccel pointer.

Fixes a warning:
fftools/ffmpeg_opt.c:804:29: warning: assignment discards ‘const’ qualifier from pointer target type

fftools/ffmpeg_opt.c

@@ -702,7 +702,8 @@ static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
         AVStream *st = ic->streams[i];
         AVCodecParameters *par = st->codecpar;
         InputStream *ist = av_mallocz(sizeof(*ist));
-        char *framerate = NULL, *hwaccel = NULL, *hwaccel_device = NULL;
+        char *framerate = NULL, *hwaccel_device = NULL;
+        const char *hwaccel = NULL;
         char *hwaccel_output_format = NULL;
         char *codec_tag = NULL;
         char *next;