examples/transcoding: suppress build warning.

suppress the "warning: assignment discards ‘const’ qualifier from
pointer target type" build warning.
Signed-off-by: Jun Zhao <jun.zhao@intel.com>
Reviewed-by: Steven Liu <lingjiujianke@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/examples/transcoding.c

@@ -227,8 +227,8 @@ static int init_filter(FilteringContext* fctx, AVCodecContext *dec_ctx,
 {
     char args[512];
     int ret = 0;
-    AVFilter *buffersrc = NULL;
-    AVFilter *buffersink = NULL;
+    const AVFilter *buffersrc = NULL;
+    const AVFilter *buffersink = NULL;
     AVFilterContext *buffersrc_ctx = NULL;
     AVFilterContext *buffersink_ctx = NULL;
     AVFilterInOut *outputs = avfilter_inout_alloc();