rpm: expose $systemd_util_dir also as rpm macro

rpm: expose $systemd_util_dir also as rpm macro

This variable (a.k.a. rootlibexecdir), was exposed through the .pc file, but
not as rpm macro.

The .pc file must be located in the package that provides libraries, which
pulls in a lot of dependencies. In Fedora, the macros are split out to a
separate package so that other packages which need to refer to some systemd
path but don't otherwise require it, can only pull in the (much smaller and
dependency-less) macros package. zram-generator uses the path to specify the
location of systemd-makefs, so by using the rpm macro we'll be able to use
a much smaller buildroot.