avfilter: Constify non-const filters

This makes the filters match their declaration in
libavfilter/allfilters.c; the earlier discrepancy was btw UB.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/vf_libplacebo.c

@@ -746,7 +746,7 @@ static const AVFilterPad libplacebo_outputs[] = {
     },
 };
 
-AVFilter ff_vf_libplacebo = {
+const AVFilter ff_vf_libplacebo = {
     .name           = "libplacebo",
     .description    = NULL_IF_CONFIG_SMALL("Apply various GPU filters from libplacebo"),
     .priv_size      = sizeof(LibplaceboContext),

libavfilter/vf_overlay_vaapi.c

@@ -408,7 +408,7 @@ static const AVFilterPad overlay_vaapi_outputs[] = {
     },
 };
 
-AVFilter ff_vf_overlay_vaapi = {
+const AVFilter ff_vf_overlay_vaapi = {
     .name            = "overlay_vaapi",
     .description     = NULL_IF_CONFIG_SMALL("Overlay one video on top of another"),
     .priv_size       = sizeof(OverlayVAAPIContext),

libavfilter/vf_siti.c

@@ -335,7 +335,7 @@ static const AVFilterPad avfilter_vf_siti_outputs[] = {
     },
 };
 
-AVFilter ff_vf_siti = {
+const AVFilter ff_vf_siti = {
     .name          = "siti",
     .description   = NULL_IF_CONFIG_SMALL("Calculate spatial information (SI) and temporal information (TI)."),
     .priv_size     = sizeof(SiTiContext),

libavfilter/vf_yadif_videotoolbox.m

@@ -440,7 +440,7 @@ static const AVFilterPad yadif_videotoolbox_outputs[] = {
     },
 };
 
-AVFilter ff_vf_yadif_videotoolbox = {
+const AVFilter ff_vf_yadif_videotoolbox = {
     .name           = "yadif_videotoolbox",
     .description    = NULL_IF_CONFIG_SMALL("YADIF for VideoToolbox frames using Metal compute"),
     .priv_size      = YADIF_VT_CTX_SIZE,