rpm: add BR: systemd for pre/post install macros

Fixes QubesOS/qubes-issues#3926

rpm_spec/core-agent.spec.in

@@ -156,6 +156,7 @@ BuildRequires: pam-devel
 %if 0%{?rhel} >= 7
 BuildRequires: python-setuptools
 %endif
+BuildRequires: systemd
 Source0: %{name}-%{version}.tar.gz
 
 %description