Include snapmgr.h in blscan.c

Windows builds on buildfarm are failing because
old_snapshot_threshold is not found in the bloom filter contrib
module.

contrib/bloom/blscan.c

@@ -19,6 +19,7 @@
 #include "storage/lmgr.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
+#include "utils/snapmgr.h"
 
 #include "bloom.h"