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: Ie551c0d3549c73c33e31ccb17149d4ad16170adb
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 74f1504..fc8ca68 100644
--- a/meson.build
+++ b/meson.build
@@ -55,7 +55,7 @@
     output: 'phosphor-virtual-sensor.service',
     configuration: conf_data,
     install: true,
-    install_dir: systemd.get_variable('systemdsystemunitdir'),
+    install_dir: systemd.get_variable('systemd_system_unit_dir'),
 )
 
 build_tests = get_option('tests')