debian/control: add iproute2 to build dependencies

debian/control: add iproute2 to build dependencies

Test scripts like qa/tasks/cephfs/mount.py expect the ip command to be
available in the container environment. Without it, tests fail with:

```
  /bin/bash: line 1: ip: command not found

  File "/ceph/qa/tasks/cephfs/mount.py", line 96, in cleanup_stale_netnses_and_bridge
    p = remote.run(args=['ip', 'netns', 'list'],
  ...
  teuthology.exceptions.CommandFailedError: Command failed with status 127: 'ip netns list'
```

Add iproute2 to the debian package build dependencies when the
<pkg.ceph.check> build profile is enabled. This ensures the package is
available during container-based builds, since buildcontainer-setup.sh
→ script/run-make.sh → install-deps.sh → debian/control → generated
dependency package chain respects build profiles configured via
`FOR_MAKE_CHECK` and `WITH_CRIMSON` environment variables set in
Dockerfile.build.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>