meson: move declarations of cgroups-agent, id128, volatile-root, and path

src/path/meson.build

@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+executables += [
+        executable_template + {
+                'name' : 'systemd-path',
+                'public' : true,
+                'sources' : files('path.c'),
+        },
+]