rpm: add BR: pkgconfig

%selinux_requires resolves to `BR: pkgconfig(systemd)` (among others), which
requires BR: pkgconfig to work.

QubesOS/qubes-issues#6567

Author: marmarek

rpm_spec/gui-agent-vm.spec.in

@@ -33,6 +33,7 @@ BuildRequires: qubes-core-agent-selinux
 Requires(post): qubes-core-agent-selinux
 BuildRequires: qubes-core-qrexec-vm-selinux
 Requires(post): qubes-core-qrexec-vm-selinux
+BuildRequires: pkgconfig
 %{?selinux_requires}
 
 Source0: qubes-gui-agent-%{version}.tar.gz