meson: switch code dependency to libsystemd

For a while now systemd has packaged a separate `libsystemd`.  For
meson dependency, `systemd` should be used when looking for systemd
service file paths and `libsystemd` should be used for `sd_*` functions.

Update the dependency accordingly.

Change-Id: Iab1cb22a31bed0814e993194a6a3383a2e5ba24b
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index d93ad19..5cd3c7a 100644
--- a/meson.build
+++ b/meson.build
@@ -33,7 +33,7 @@
         dependency('phosphor-dbus-interfaces'),
         dependency('phosphor-logging'),
         dependency('sdbusplus'),
-        dependency('systemd'),
+        dependency('libsystemd'),
         dependency('openssl'),
         dependency('nlohmann_json',include_type: 'system'),
 ]