pgsql: Add missing .gitignore files.

Add missing .gitignore files.

contrib/pg_stash_advice and src/test/modules/test_shmem
missed these, leading to complaints from git after an
in-tree check-world run.

Use our standard boilerplate list of ignorable subdirectories,
although the two modules presently create different subsets
of that.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f7da81f68b63430d24caba99c0a95933c10cefcb

Modified Files
--------------
contrib/pg_stash_advice/.gitignore     | 4 ++++
src/test/modules/test_shmem/.gitignore | 4 ++++
2 files changed, 8 insertions(+)