Resolve invalid-pkgconfig-file rpmlint error

systemd 216 systemd.pc has references to pkg-config variables in the form
of ${whatever} variables and this causes the rpmlint error above, which
breaks the build for excessive badness.

rpmlintrc set badness to 0 for invalid-pkgconfig-file, but unfortunately
Mer OBS ignores rpmlintrc and nobody seems to care, so we have to be
creative here.

Expand variables in systemd.pc.in to actual values, that should make
rpmlint happy.