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: I4b91c906724cd52534dc00fd213edfd0e5066088
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 5d8e3fd..4bd7365 100644
--- a/meson.build
+++ b/meson.build
@@ -85,7 +85,7 @@
 endif
 
 
-servicedir = systemd_dep.get_variable('systemdsystemunitdir')
+servicedir = systemd_dep.get_variable('systemd_system_unit_dir')
 usr_share_dir = '/usr/share/phosphor-fan-presence'
 
 conf = configuration_data()