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: I316527fa798ab5acc8569f2e8de932fffdde1070
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/service_files/meson.build b/service_files/meson.build
index baac30f..5b949a2 100644
--- a/service_files/meson.build
+++ b/service_files/meson.build
@@ -1,6 +1,6 @@
 systemd = dependency('systemd')
 systemd_system_unit_dir = systemd.get_variable(
-    'systemdsystemunitdir',
+    'systemd_system_unit_dir',
     pkgconfig_define: ['prefix', get_option('prefix')],
 )