BuildRequires libtirpc-devel needed for RHEL 8

Building against RHEL 8 requires libtirpc-devel, as with fedora 28.
Add rhel8 and centos8 options to the test, to account for that.

BuildRequires Originally added for fedora 28 via commit
1a62a305be01972ef1b81469134faa4937836096

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #9289