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: I63dbeffffe3470904df0fc18d337fb2c40cc72b9
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 549b4df..9a8b727 100644
--- a/meson.build
+++ b/meson.build
@@ -83,7 +83,7 @@
         'DEFAULT_BUSNAME': default_busname,
     },
     install: true,
-    install_dir: dependency('systemd').get_variable('systemdsystemunitdir'),
+    install_dir: dependency('systemd').get_variable('systemd_system_unit_dir'),
 )
 
 configure_file(
diff --git a/src/ibm/hypervisor-network-mgr-src/meson.build b/src/ibm/hypervisor-network-mgr-src/meson.build
index 22b82c6..d081f32 100644
--- a/src/ibm/hypervisor-network-mgr-src/meson.build
+++ b/src/ibm/hypervisor-network-mgr-src/meson.build
@@ -8,7 +8,7 @@
         'HYP_DEFAULT_NETWORK_BUSNAME': hyp_default_busname,
     },
     install: true,
-    install_dir: dependency('systemd').get_variable('systemdsystemunitdir'),
+    install_dir: dependency('systemd').get_variable('systemd_system_unit_dir'),
 )
 
 hyp_src_includes = include_directories('.')