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.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Iff3e5cc93a503e7391e4eea9624b04069135a42a
diff --git a/meson.build b/meson.build
index 4bd4c3f..f682ca4 100644
--- a/meson.build
+++ b/meson.build
@@ -102,7 +102,6 @@
         phosphor_dbus_interfaces,
         phosphor_logging,
         sdbusplus,
-        dependency('systemd'),
         dependency('threads'),
         dependency('tinyxml2', default_options: ['tests=false']),
     ],