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: Id6794485cb085fedc3c9efcc8c34a5faa1beac97
diff --git a/meson.build b/meson.build
index 946aaef..aaaf988 100644
--- a/meson.build
+++ b/meson.build
@@ -51,7 +51,7 @@
     'src/zlib_file.cpp',
   ],
   dependencies: [
-    systemd,
+    dependency('libsystemd'),
     dependency('phosphor-logging'),
     dependency('zlib'),
   ],