pg_dump: Tiny header cleanup

pg_dump: Tiny header cleanup

In commits 9c02e3a986da and 8ec0aaeae094, Nathan added a duplicate
TocEntry typedef forward declaration (plus assorted #ifdef hackery to
avoid C99 preprocessor issues) to deal with some very old untidyness
regarding DefnDumperPtr function prototype being located in pg_backup.h.
But there's no reason to have the DefnDumperPtr typedef (and the
accompanying DataDumperPtr typedef) in that file at all; they are better
placed in pg_backup_archiver.h, the internal header, because they are
only used internally.  That also requires zero #ifdef hackery, so move
them there.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/202504042140.qo66ggw6wzsz@alvherre.pgsql