lavfi/tests/filtfmts: Constify a variable.

Fixes the following warning:
libavfilter/tests/filtfmts.c: In function ‘main’:
libavfilter/tests/filtfmts.c:103:18: warning: assignment discards ‘const’ qualifier from pointer target type

libavfilter/tests/filtfmts.c

@@ -73,7 +73,7 @@ static void print_formats(AVFilterContext *filter_ctx)
 
 int main(int argc, char **argv)
 {
-    AVFilter *filter;
+    const AVFilter *filter;
     AVFilterContext *filter_ctx;
     AVFilterGraph *graph_ctx;
     const char *filter_name;