Avoid duplicate file references in backend .pot file

The list of files for the backend's message catalog is collected with
find, which also lists symlinks, namely the generated files that are
linked from src/include/utils/ to src/backend/utils/ (such as
guc_tables.inc.c) and the port/pg_sema.c and port/pg_shmem.c links
created by configure. Since the link targets are found under their real
names as well, xgettext processed those files twice, resulting in
duplicate "#:" references like

#: ../include/utils/guc_tables.inc.c:410 utils/guc_tables.inc.c:410

This is an old problem, but the introduction of guc_tables.inc.c in PG19
has made it much more noticeable.

I propose the attached patch to fix this by restricting find to regular
files.