meson: use non-deprecated systemd packageconfig

Systemd's packageconfig file has both `systemdsystemunitdir` and
`systemd_system_unit_dir` defined.  The non-underscore one appears
to be a deprecated alias[1].  Move to the non-deprecated /
underscore-separated variant.

[1]: https://github.com/systemd/systemd/commit/4908de44b0a0409f84a7cdc5641b114d6ce8ba03

Change-Id: I4d1b59b0203ade1e61c67ec175d59a00ccd4f184
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 9030537..471def6 100644
--- a/meson.build
+++ b/meson.build
@@ -61,7 +61,7 @@
         'service_files/chassis-system-reset.service',
         'service_files/chassis-system-reset.target',
         'service_files/xyz.openbmc_project.Chassis.Control.Power@.service',
-        install_dir: systemd.get_variable('systemdsystemunitdir'),
+        install_dir: systemd.get_variable('systemd_system_unit_dir'),
     )
 endif