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: I4d93cd8cf70305377ba4eb336ea9a18ba482abf0
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 7e02851..db3f57f 100644
--- a/meson.build
+++ b/meson.build
@@ -50,7 +50,7 @@
 
 systemd = dependency('systemd')
 systemd_system_unit_dir = systemd.get_variable(
-    'systemdsystemunitdir',
+    'systemd_system_unit_dir',
     pkgconfig_define: ['prefix', get_option('prefix')],
 )