Fixes <https://issues.guix.gnu.org/48521>.
The problem was caused by the 'file-system-mapping' record not being in scope.
* gnu/services/networking.scm (opendht-shepherd-service): Import the (gnu
system file-systems) module.
[requirement]: Depend on networking, to avoid spurious output.
[modules]: New field.
[start] <group>: New argument.