build: bump once_cell from 1.12.0 to 1.12.1

Bumps [once_cell](https://github.com/matklad/once_cell) from 1.12.0 to 1.12.1.
- [Release notes](https://github.com/matklad/once_cell/releases)
- [Changelog](https://github.com/matklad/once_cell/blob/master/CHANGELOG.md)
- [Commits](https://github.com/matklad/once_cell/compare/v1.12.0...v1.12.1)

---
updated-dependencies:
- dependency-name: once_cell
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

net_util/Cargo.toml

@@ -21,7 +21,7 @@ vm-virtio = { path = "../vm-virtio" }
 vmm-sys-util = "0.9.0"
 
 [dev-dependencies]
-once_cell = "1.12.0"
+once_cell = "1.12.1"
 pnet = "0.31.0"
 pnet_datalink = "0.31.0"
 serde_json = "1.0.82"