linux-postmarketos-qcom-msm8974: compile modules for netboot and iptables 'statistic'

I managed to get a couple of fairphone-fp2s with mainline kernel to join a k8s cluster using the standard Alpine-packaged 1.25 kubeadm + kubectl + containerd packages. The only thing that appears to have been missing thus far is this xt_statistic module which kube-proxy seems to rely on for ... whatever it is kube-proxy does exactly ...

I also added CONFIG_BLK_DEV_NBD=m since it was the only thing pmbootstrap kconfig check postmarketos-qcom-msm8974 --netboot seemed concerned about. I haven't actually tried it yet with this kernel.