syncthing: fix build (#51866)

###### Motivation for this change

fixes #51825 

###### Things done

<!-- Please check what applies. Note that these are not hard requirements but merely serve as information for reviewers. -->

- [x] Tested using sandboxing ([nix.useSandbox](http://nixos.org/nixos/manual/options.html#opt-nix.useSandbox) on NixOS, or option `sandbox` in [`nix.conf`](http://nixos.org/nix/manual/#sec-conf-file) on non-NixOS)
- Built on platform(s)
   - [x] NixOS
   - [ ] macOS
   - [ ] other Linux distributions
- [ ] Tested via one or more NixOS test(s) if existing and applicable for the change (look inside [nixos/tests](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests))
- [ ] Tested compilation of all pkgs that depend on this change using `nix-shell -p nox --run "nox-review wip"`
- [ ] Tested execution of all binary files (usually in `./result/bin/`)
- [ ] Determined the impact on package closure size (by running `nix path-info -S` before and after)
- [ ] Assured whether relevant documentation is up to date
- [x] Fits [CONTRIBUTING.md](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md).

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