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: Ifa1dcc50959c2b2f5bda469c6a75a2e0be7534e5
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index c5d05e3..0f5117e 100644
--- a/meson.build
+++ b/meson.build
@@ -128,7 +128,7 @@
 meson.override_find_program('mapperx', mapperx)
 
 systemd_system_unit_dir = dependency('systemd').get_variable(
-    'systemdsystemunitdir',
+    'systemd_system_unit_dir',
 )
 
 conf = configuration_data()