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: I09b62768774ae85d1352802a9bf571049283bfc9
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 02ea2f2..48a03a9 100644
--- a/meson.build
+++ b/meson.build
@@ -38,11 +38,11 @@
 systemd = dependency('systemd')
 install_data(
     ['systemd' / 'system' / 'phosphor-ledcontroller.service'],
-    install_dir: systemd.get_variable(pkgconfig: 'systemdsystemunitdir'),
+    install_dir: systemd.get_variable(pkgconfig: 'systemd_system_unit_dir'),
 )
 install_data(
     ['systemd' / 'system' / 'sysfs-led@.service'],
-    install_dir: systemd.get_variable(pkgconfig: 'systemdsystemunitdir'),
+    install_dir: systemd.get_variable(pkgconfig: 'systemd_system_unit_dir'),
 )
 
 dbus = dependency('dbus-1')