meson: Add optional libsystemd dependency

To utilize the API provided by libsystemd it would be better to
create a separate HAVE_LIBSYSTEMD configuration option instead of
having to rely on HAVE_NATIVE_BACKEND.

For now this will be utilized for getting the control group of a
MetaWindow.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1960>

src/meson.build

@@ -118,6 +118,12 @@ if have_wayland
   ]
 endif
 
+if have_libsystemd
+  mutter_pkg_private_deps += [
+    libsystemd_dep,
+  ]
+endif
+
 if have_native_backend
   mutter_pkg_private_deps += [
     libdrm_dep,