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: I9dc6dd74a13b874a026b673578c5ce5331d0e419
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 5f59b8a..7f55a2e 100644
--- a/meson.build
+++ b/meson.build
@@ -151,7 +151,7 @@
 install_headers(libmctp_headers)
 
 if systemd_dep.found()
-    unitdir = systemd_dep.get_variable(pkgconfig: 'systemdsystemunitdir')
+    unitdir = systemd_dep.get_variable(pkgconfig: 'systemd_system_unit_dir')
     install_data('systemd/system/mctp-demux.service', install_dir: unitdir)
     install_data('systemd/system/mctp-demux.socket', install_dir: unitdir)
 endif