syslog-ng: expand service file @variables@ earlier

Move variable expansion of @SBINDIR@, @LOCALSTATEDIR@, and @BASEBINDIR@
earlier during install, to before the systemd service file is installed
by an automake rule.

Signed-off-by: Ian Ray <ian.ray@ge.com>
Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>