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: I721645f212b16a46eac3d5ef1e7111b830d88394
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 3ac77a5..d9412c6 100644
--- a/meson.build
+++ b/meson.build
@@ -28,7 +28,9 @@
 gpiodcxx = dependency('libgpiodcxx', default_options: ['bindings=cxx'])
 
 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',
+)
 
 boost = dependency(
     'boost',