lavfi: fix signature for default_filter_name()

Return a const char *, fix warnings:
libavfilter/avfilter.c: In function ‘default_filter_name’:
libavfilter/avfilter.c:414:5: warning: return discards ‘const’ qualifier from pointer target type [enabled by default]
libavfilter/avfilter.c: At top level:
libavfilter/avfilter.c:419:5: warning: initialization from incompatible pointer type [enabled by default]
libavfilter/avfilter.c:419:5: warning: (near initialization for ‘avfilter_class.item_name’) [enabled by default]

libavfilter/avfilter.c

@@ -410,7 +410,7 @@ static int pad_count(const AVFilterPad *pads)
     return count;
 }
 
-static char *default_filter_name(void *filter_ctx)
+static const char *default_filter_name(void *filter_ctx)
 {
     AVFilterContext *ctx = filter_ctx;
     return ctx->name ? ctx->name : ctx->filter->name;