filtfmts-test: Mark filter as const

libavfilter/tests/filtfmts.c:57:18: warning:
      assigning to 'AVFilter *' (aka 'struct AVFilter *') from
      'const AVFilter *' (aka 'const struct AVFilter *') discards qualifiers
      [-Wincompatible-pointer-types-discards-qualifiers]

libavfilter/tests/filtfmts.c

@@ -28,7 +28,7 @@
 
 int main(int argc, char **argv)
 {
-    AVFilter *filter;
+    const AVFilter *filter;
     AVFilterContext *filter_ctx;
     AVFilterGraph *graph_ctx;
     const char *filter_name;