Fix build tags for collectors

[ghost]

Existing build tags for collectors seems out of sync, and when I try to build for freebsd with build tag noboottime noexec nomeminfo nonetdev nozfs nofilesystem just fails with:

github.com/prometheus/node_exporter/collector/zfs_freebsd.go:24:12: undefined: bsdSysctl
github.com/prometheus/node_exporter/collector/zfs_freebsd.go:38:14: undefined: bsdSysctl

which is not the expected behavior I suppose, because zfs was included even with nozfs tag.

This PR is an attempt to fix collectors' build tags, and once merged, users should be able to build with/without specific collectors with correct build tags.

/cc @SuperQ @discordianfish

[SuperQ]

LGTM, thanks for the cleanup!