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: I8965f6d111a074a6e348baeebe0b45249037ffbc
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 945886f..6fa74c7 100644
--- a/meson.build
+++ b/meson.build
@@ -268,7 +268,7 @@
 
 if get_option('systemd').allowed()
     systemd_system_unit_dir = dependency('systemd').get_variable(
-        'systemdsystemunitdir',
+        'systemd_system_unit_dir',
     )
     filesystem.copyfile(
         'pldmd/service_files/pldmd.service',