n-dhcp4: avoid {net,linux}/if.h clashes on old distros

In particular, avoid including linux/netdevice.h from headers. This is
not a problem on newer distros, but required for CentOS 7.6.

Signed-off-by: Tom Gundersen <teg@jklm.no>
12 jobs for nettools in 73 minutes and 8 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
failed #339062
allowed to fail
checkpatch

00:01:37

passed #339067
allowed to fail manual
t_centos:7.5.1804

00:30:10

passed #339068
t_centos:7.6.1810

00:26:51

passed #339071
t_debian:9

00:20:40

passed #339073
t_debian:sid

00:19:36

passed #339072
allowed to fail manual
t_debian:testing

00:17:06

passed #339063
t_fedora:28

00:29:31

passed #339064
t_fedora:29

00:38:56

passed #339065
t_fedora:30

00:38:39

passed #339066
allowed to fail manual
t_fedora:rawhide

00:34:09

passed #339069
t_ubuntu:16.04

00:20:18

passed #339070
t_ubuntu:18.04

00:19:37

 
Name Stage Failure
failed
checkpatch Test

src/dhcp/nm-dhcp-nettools.c:1246: XXX marker are reserved for development while work-in-progress. Use TODO or FIXME comment instead?:
> /* XXX: select hostname/FQDN */

The patch does not validate.
Uploading artifacts...
checkpatch-out.txt: found 1 matching files
Uploading artifacts to coordinator... ok
id=339062 responseStatus=201 Created token=mjHiGpC_
ERROR: Job failed: exit code 1