disable nss features on musl

The musl C library does not support nss. There is no point in building any of the libnss-* packages. This was discussed in the upstream#9596 bug concluding that skipping nss libraries for musl would be best, so this is what I am proposing here.

I note that you cannot actually build systemd on Debian's musl yet. One reason is that systemd requires printf.h which Debian's musl does not provide. Such problems are going to be solved elsewhere.

In principle, it would be nice to change the Architecture field of the libnss-* packages to reflect this, but it is difficult to express. We would like to say "linux architectures, but not musl architectures". We cannot do that. An architecture list would be annoying to maintain here. Saying gnu-linux-any would be affecting other C libraries such as uclibc, so I suggest leaving it like that while the conditional in d/rules provides an immediate benefit at (imo) reasonable maintenance cost.