libplacebo: future-proof against libplacebo v6

This field was deprecated in v4 and will therefore be dropped in
libplacebo v6. Suppress the deprecation warning and future-proof by
guarding the one line that still uses it.

modules/video_output/libplacebo/display.c

@@ -346,7 +346,9 @@ static void PictureRender(vout_display_t *vd, picture_t *pic,
         target.color.primaries = sys->target.primaries;
     if (sys->target.transfer) {
         target.color.transfer = sys->target.transfer;
+#if PL_API_VER < 189
         target.color.light = PL_COLOR_LIGHT_UNKNOWN; // re-infer
+#endif
     }
     if (sys->dither_depth > 0) {
         // override the sample depth without affecting the color encoding