Fix WAL_DEBUG build

broken by commit e18512c

Reported-by: Peter Geoghegan <pg@bowt.ie>

Author: petere

src/backend/access/transam/xlog.c

@@ -104,6 +104,10 @@
 #include "utils/timestamp.h"
 #include "utils/varlena.h"
 
+#ifdef WAL_DEBUG
+#include "utils/memutils.h"
+#endif
+
 /* timeline ID to be used when bootstrapping */
 #define BootstrapTimeLineID		1