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: I92176ca17f764e3e2e1e2e864115ab098b800ea4
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/bmc/firmware-handler/meson.build b/bmc/firmware-handler/meson.build
index d544a24..9f89928 100644
--- a/bmc/firmware-handler/meson.build
+++ b/bmc/firmware-handler/meson.build
@@ -57,7 +57,7 @@
             output: data,
             configuration: conf_data,
             install: true,
-            install_dir: systemd.get_variable('systemdsystemunitdir'),
+            install_dir: systemd.get_variable('systemd_system_unit_dir'),
         )
     endforeach
 endif