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: Id8a854fed33d86416fa4ea7fbb46544e5d4682aa
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 58f9416..c9ec694 100644
--- a/meson.build
+++ b/meson.build
@@ -18,7 +18,7 @@
 
     systemd = dependency('systemd', required: true)
     systemd_system_unit_dir = systemd.get_variable(
-        pkgconfig: 'systemdsystemunitdir',
+        pkgconfig: 'systemd_system_unit_dir',
     )
 
     boost = dependency('boost', version: '>=1.82')