BUG #15859: Postgres 11 repository is missing extensions

The following bug has been logged on the website:

Bug reference: 15859
Logged by: Runali Patel
Email address: runali(dot)patel(at)hibu(dot)com
PostgreSQL version: 11.3
Operating system: Centos 6.9
Description:

Postgres 11 repository is missing extensions like pg_stat_statements and
many others after the initial install. It contains only one extension:
plpgsql

postgres=# select * from pg_available_extensions;
name | default_version | installed_version | comment

---------+-----------------+-------------------+------------------------------
plpgsql | 1.0 | 1.0 | PL/pgSQL procedural
language
(1 row)