rpm: Ensure %{systemd_requires} availability

RPM macros for systemd used to be part of the main systemd package but
on Fedora they moved to their own subpackage. The %{systemd_requires}
macro is no longer guaranteed to be present in a default build root.

This change enables Fedora Rawhide builds, for a current Rawhide target
of Fedora 36.

Closes #153

redhat/varnish.spec

@@ -25,10 +25,13 @@ BuildRequires: pcre2-devel
 BuildRequires: pkgconfig
 BuildRequires: python3
 BuildRequires: python3-sphinx
+%if 0%{?fedora}
+BuildRequires: systemd-rpm-macros
+%endif
 
 Requires: gcc
 Requires: logrotate
-%systemd_requires
+%{?systemd_requires}
 %if 0%{?rhel} >= 8
 Requires: redhat-rpm-config
 %endif