pgsql: Don't include execnodes.h in replication/conflict.h

Don't include execnodes.h in replication/conflict.h

... which silently propagates a lot of headers into many places
via pgstat.h, as evidenced by the variety of headers that this patch
needs to add to seemingly random places.  Add a minimum of typedefs to
conflict.h to be able to remove execnodes.h, and fix the fallout.

Backpatch to 18, where conflict.h first appeared.

Discussion: https://postgr.es/m/202509191927.uj2ijwmho7nv@alvherre.pgsql

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/7e638d7f5093fd24837ebe709135fa16ef8e3e7b

Modified Files
--------------
src/backend/access/transam/multixact.c      | 1 +
src/backend/access/transam/xlogrecovery.c   | 1 +
src/backend/storage/ipc/waiteventset.c      | 1 +
src/backend/utils/activity/pgstat_backend.c | 1 +
src/include/pgstat.h                        | 1 +
src/include/replication/conflict.h          | 9 ++++++++-
6 files changed, 13 insertions(+), 1 deletion(-)