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: Ieec38724a82e2f403b105dd201969fd36d3241cb
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index d11437a..c7d3c95 100644
--- a/meson.build
+++ b/meson.build
@@ -105,7 +105,7 @@
 
 systemd = dependency('systemd')
 libsystemd_dep = dependency('libsystemd')
-servicedir = systemd.get_variable('systemdsystemunitdir')
+servicedir = systemd.get_variable('systemd_system_unit_dir')
 
 services = [
     ['supply-monitor', 'power-supply-monitor@.service'],