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: I0ae94b17ccfe82c1fbc3e4e0602b0a6ecd21762d
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index f1a5428..0bc6b7c 100644
--- a/meson.build
+++ b/meson.build
@@ -60,7 +60,7 @@
 ssl = dependency('openssl')
 
 systemd = dependency('systemd')
-systemd_system_unit_dir = systemd.get_variable('systemdsystemunitdir')
+systemd_system_unit_dir = systemd.get_variable('systemd_system_unit_dir')
 
 build_tests = get_option('tests')