Change type of rd variable

Silences some warnings.

Change-Id: I8e90368a973c1fc4a88a1bb92b779aa68981dbb5
diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 52c54d8..bda9cff 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -7657,7 +7657,7 @@
   int rate;
   uint64_t sse;
   int64_t dist;
-  int rd0;
+  int64_t rd0;
   SEG_MASK_TYPE cur_mask_type;
   int64_t best_rd = INT64_MAX;
   SEG_MASK_TYPE best_mask_type = 0;