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: I5b5c4ab5a22c5263018357eed81482638d133cc2
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 40ab5d9..9b0040f 100644
--- a/meson.build
+++ b/meson.build
@@ -148,7 +148,7 @@
 )
 
 
-systemd_system_unit_dir = systemd_dep.get_variable('systemdsystemunitdir')
+systemd_system_unit_dir = systemd_dep.get_variable('systemd_system_unit_dir')
 
 install_data(
     'phosphor-nslcd-cert-config.conf',