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
1a62a30

rpm/generic/zfs.spec.in

@@ -139,7 +139,7 @@ BuildRequires:  libblkid-devel
 BuildRequires:  libudev-devel
 BuildRequires:  libattr-devel
 BuildRequires:  openssl-devel
-%if 0%{?fedora} >= 28
+%if 0%{?fedora} >= 28 || 0%{?rhel} >= 8 || 0%{?centos} >= 8
 BuildRequires:  libtirpc-devel
 %endif
 Requires:       openssl