Fix a warning about fallthrough in 'switch'

Introduced by this patch:
https://aomedia.googlesource.com/aom/+/bf0b45a089d8b775236b3ab3a7cfc3d78dda39c1

Change-Id: I56b71a592f509179e2834dcd8cfa60f55d2460e0
diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 218cda2..6e6faa0 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -8198,6 +8198,7 @@
                             INT64_MAX);
       PrintPredictionUnitStats(cpi, tile_data, x, &rd_stats_y, bsize);
 #endif  // CONFIG_COLLECT_RD_STATS == 3
+      AOM_FALLTHROUGH_INTENDED;
     case INTERP_SKIP_LUMA_EVAL_CHROMA:
       // skip_pred = 1: skip luma evaluation (retain previous best luma stats)
       // and do chroma evaluation.