avfilter/af_atilt: Constify filter

(It is actually UB if a declaration and its definition differ wrt
their types like they do in this case (the declaration in allfilters
is const).)

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/af_atilt.c

@@ -253,7 +253,7 @@ static const AVFilterPad outputs[] = {
     },
 };
 
-AVFilter ff_af_atilt = {
+const AVFilter ff_af_atilt = {
     .name            = "atilt",
     .description     = NULL_IF_CONFIG_SMALL("Apply spectral tilt to audio."),
     .priv_size       = sizeof(ATiltContext),