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: I1980482e7b40e36f7bd9dd524ef678b60374b670
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/transport/serialbridge/meson.build b/transport/serialbridge/meson.build
index 6919c75..36339b7 100644
--- a/transport/serialbridge/meson.build
+++ b/transport/serialbridge/meson.build
@@ -32,7 +32,7 @@
         output: 'serialbridge@.service',
         configuration: conf_data,
         install: true,
-        install_dir: systemd.get_variable(pkgconfig: 'systemdsystemunitdir'),
+        install_dir: systemd.get_variable(pkgconfig: 'systemd_system_unit_dir'),
     )
 endif