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.patel@hibu.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)