examples/filtering_audio: suppress the 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/filtering_audio.c

@@ -89,8 +89,8 @@ static int init_filters(const char *filters_descr)
 {
     char args[512];
     int ret = 0;
-    AVFilter *abuffersrc  = avfilter_get_by_name("abuffer");
-    AVFilter *abuffersink = avfilter_get_by_name("abuffersink");
+    const AVFilter *abuffersrc  = avfilter_get_by_name("abuffer");
+    const AVFilter *abuffersink = avfilter_get_by_name("abuffersink");
     AVFilterInOut *outputs = avfilter_inout_alloc();
     AVFilterInOut *inputs  = avfilter_inout_alloc();
     static const enum AVSampleFormat out_sample_fmts[] = { AV_SAMPLE_FMT_S16, -1 };