Fix thinko in previous commit

Since postgres.h includes palloc.h, definitions that affect the latter
must be present before the former is included.

Per buildfarm results

src/backend/utils/mmgr/mcxt.c

@@ -19,11 +19,11 @@
  *-------------------------------------------------------------------------
  */
 
-#include "postgres.h"
-
-/* see palloc.h */
+/* see palloc.h.  Must be before postgres.h */
 #define MCXT_INCLUDE_DEFINITIONS
 
+#include "postgres.h"
+
 #include "utils/memutils.h"