(packaging) Move systemd BuildRequires into conditional

Previously either systemd or systemd-units were BuildRequires on all rpms.
These packages are not available on el5 or el6, which would break rpm building
on those platforms. This commit moves those BuildRequires into an already
existing systemd conditional, so they are only required on systems that support
systemd.
Merge pull request #1790 from haus/update_redhat_spec

(packaging) Move systemd BuildRequires into conditional