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: I862b6b947059cc5955990a819578e8984d2b52e2
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/attn/meson.build b/attn/meson.build
index 674e302..9bb0b78 100644
--- a/attn/meson.build
+++ b/attn/meson.build
@@ -27,7 +27,7 @@
     input: 'attn_handler.service.in',
     output: 'attn_handler.service',
     install: true,
-    install_dir: dependency('systemd').get_variable('systemdsystemunitdir'),
+    install_dir: dependency('systemd').get_variable('systemd_system_unit_dir'),
 )
 
 # install attention handler config file