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: I7bf9b444aa8f04eb2b8ded05e502fee47e903639
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index e92821f..23e3f28 100644
--- a/meson.build
+++ b/meson.build
@@ -351,7 +351,7 @@
     endforeach
 endif
 
-systemd_system_unit_dir = systemd.get_variable('systemdsystemunitdir')
+systemd_system_unit_dir = systemd.get_variable('systemd_system_unit_dir')
 
 bindir = get_option('prefix') + '/' + get_option('bindir')
 libexec = get_option('prefix') + '/' + get_option('libexecdir')