Accepting request 702584 from GNOME:Next

- Replace systemd-gtk BuildRequires with pkgconfig(systemd): make
  the build cheaper by not having to wait for the 'real' systemd
  package to have built, but allow to use systemd-mini. The change
  in the stack causing this was polkit dropping its hard dep on
  systemd.

- Add systemd-gtk BuildRequires: Needed now after changes elsewhere
  in the stack.

OBS-URL: https://build.opensuse.org/request/show/702584
OBS-URL: https://build.opensuse.org/package/show/GNOME:Factory/flatpak?expand=0&rev=76

flatpak.spec

@@ -52,6 +52,7 @@ BuildRequires:  pkgconfig(libseccomp)
 BuildRequires:  pkgconfig(libsoup-2.4)
 BuildRequires:  pkgconfig(ostree-1) >= 2018.7
 BuildRequires:  pkgconfig(polkit-gobject-1)
+BuildRequires:  pkgconfig(systemd)
 BuildRequires:  pkgconfig(xau)
 Requires:       %{libname} = %{version}
 Requires:       bubblewrap >= 0.2.1