Fix BTREE_BUILD_STATS build.

Commit 65c5fcd3 broke this by removing a
header include directive that is conditionally required.  Add that back
to nbtree.c, with annotation to keep pgrminclude from re-breaking it.

Peter Geoghegan

Report: <CAM3SWZTNjHFYW_UG8bu0BnogqQ2HfsTgkzXLueuUhfTcYbu5HA@mail.gmail.com>

src/backend/access/nbtree/nbtree.c

@@ -27,6 +27,7 @@
 #include "storage/ipc.h"
 #include "storage/lmgr.h"
 #include "storage/smgr.h"
+#include "tcop/tcopprot.h"		/* pgrminclude ignore */
 #include "utils/index_selfuncs.h"
 #include "utils/memutils.h"