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: I32b2ac4d38b943e6a3567b1b695635fdf4e68951
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 933c1d9..01e10aa 100644
--- a/meson.build
+++ b/meson.build
@@ -23,7 +23,9 @@
 sdbusplus = dependency('sdbusplus')
 
 systemd = dependency('systemd', required: true)
-systemd_system_unit_dir = systemd.get_variable(pkgconfig: 'systemdsystemunitdir')
+systemd_system_unit_dir = systemd.get_variable(
+    pkgconfig: 'systemd_system_unit_dir',
+)
 
 peci = dependency('libpeci')