collector: add NetBSD support for netdev

[tnn2]

It's similar to the other BSD collectors but we have to do a special sysctl dance to read ifi_oqdrops.

[discordianfish]

Build fails:

# github.com/prometheus/node_exporter/collector
collector/netdev_netbsd.go:33:10: fatal error: sys/sysctl.h: No such file or directory
   33 | #include <sys/sysctl.h>
      |          ^~~~~~~~~~~~~~
compilation terminated.
!! command failed: build -o .build/netbsd-386/node_exporter -ldflags -X github.com/prometheus/common/version.Version=1.8.1 -X github.com/prometheus/common/version.Revision=e8dd999618cea83b8213e5a63fe956b5ecb4fe6d -X github.com/prometheus/common/version.Branch=pull/3078 -X github.com/prometheus/common/version.BuildUser=root@d3958b0be1a1 -X github.com/prometheus/common/version.BuildDate=20240728-00:51:43  -extldflags '-static' github.com/prometheus/node_exporter: exit status 1
make: *** [Makefile.common:204: common-build] Error 1
!! The main builder docker image exited unexpectedly: exit status 2

Exited with code exit status 1

[tnn2]

collector/netdev_netbsd.go:33:10: fatal error: sys/sysctl.h: No such file or directory
33 | #include <sys/sysctl.h>
| ^~~~~~~~~~~~~~
compilation terminated.

I can't reproduce this. sys/sysctl.h is definitely a file that should exist.
It might be a problem with the build environment but I cannot say
what the problem is. I noticed the build log has

CC='i686-linux-gnu-gcc'

Does this job even build a NetBSD binary?

[discordianfish]

@tnn2 this is from crosscompiling using promu:

• https://github.com/prometheus/promu

Using this config: https://github.com/prometheus/node_exporter/blob/master/.promu-cgo.yml