Fix missing installation rules for BackgroundPsql.pm
Commit
664d75753 added the BackgroundPsql module with helper functions
for tests running interactive or background psql tasks. The new module
was however not added to the install rules of the build systems.
Reported-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/
c0ba3008-dbc8-e53f-29f2-
2e9abe72b2a2@enterprisedb.com