Fix compile warning w/ -DCONFIG_COLLECT_RD_STATS=1

BUG=aomedia:2220

Change-Id: Ia833f5d8b722fffee077e0d97c43b0727d78baad
diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 964d5f7..5dd0d59 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -2494,7 +2494,7 @@
   const int dequant_shift =
       (xd->cur_buf->flags & YV12_FLAG_HIGHBITDEPTH) ? xd->bd - 5 : 3;
   const int q_step = pd->dequant_Q3[1] >> dequant_shift;
-  const double num_samples = txw * txh;
+  const int num_samples = txw * txh;
 
   const double rate_norm = (double)rd_stats->rate / num_samples;
   const double dist_norm = (double)rd_stats->dist / num_samples;