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: I6d55fcb9af74863b3f3905d787d54f67557abc76
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index a94cb4b..ecea2be 100644
--- a/meson.build
+++ b/meson.build
@@ -51,6 +51,6 @@
 if systemd.found()
     install_data(
         'service_files/xyz.openbmc_project.Logging.IPMI.service',
-        install_dir: systemd.get_variable('systemdsystemunitdir'),
+        install_dir: systemd.get_variable('systemd_system_unit_dir'),
     )
 endif