Declare ms_stat inside #if COLLECT_PICK_MODE_STAT

The global variable ms_stat is only used in #if COLLECT_PICK_MODE_STAT
blocks, so declare the variable inside #if COLLECT_PICK_MODE_STAT.

A follow-up to https://aomedia-review.googlesource.com/c/aom/+/167742.

Change-Id: Ia1e32e8e066556cc4328bf29ce1114456df15925

av1/encoder/nonrd_pickmode.c

diff --git a/av1/encoder/nonrd_pickmode.c b/av1/encoder/nonrd_pickmode.c
index 4e6b3a9..e62cac1 100644
--- a/av1/encoder/nonrd_pickmode.c
+++ b/av1/encoder/nonrd_pickmode.c
@@ -2323,7 +2323,9 @@
   struct aom_usec_timer bsize_timer;
 } mode_search_stat;
 
+#if COLLECT_PICK_MODE_STAT
 static mode_search_stat ms_stat;
+#endif
 
 static AOM_INLINE void print_stage_time(const char *stage_name,
                                         int64_t stage_time,