pgsql: Fix dependency on FDW handler.

Fix dependency on FDW handler.

ALTER FOREIGN DATA WRAPPER could drop the dependency on the handler
function if it wasn't explicitly specified.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/35c44a4b7fb76d35418c4d66b775a88f4ce60c86.camel@j-davis.com
Backpatch-through: 14

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/703fee3b25c45b3b336b95f91a6a89ade85815f4

Modified Files
--------------
src/backend/commands/foreigncmds.c         | 5 +++++
src/test/regress/expected/foreign_data.out | 7 +++++++
src/test/regress/sql/foreign_data.sql      | 5 +++++
3 files changed, 17 insertions(+)