pgsql: Fix missing installation/uninstallation rules for BackgroundPsql

Fix missing installation/uninstallation rules for BackgroundPsql.pm

Commit d5fd7865 backported BackgroundPsql perl module module with
helper functions for tests running interactive or background psql
tasks to PG 12 to 15, but did not add installation/uninstallation
rules of the build system, causing problems running TAP tests for the
extensions.

Author: Pavan Deolasee <pavan.deolasee@gmail.com>
Discussion:
https://www.postgresql.org/message-id/CABOikdPmRuZrcf_gtgXmQzZ5Tbg9yUJmqXDCAZ2aW%3DWi-PbDyQ%40mail.gmail.com

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/1185be355462d1dc7e2950a7e52eb7ca0cb6f3c8

Modified Files
--------------
src/test/perl/Makefile | 2 ++
1 file changed, 2 insertions(+)