[PATCH] nixos/modules/unbound: include AF_NETLINK in permitted AFs

As of unbound 1.12.0 the AF_NETLINK is required during process startup
in some configurations. Without this patch the process startup
terminates as follows:

[1607371992] unbound[10523:0] error: failed to list interfaces: getifaddrs: Address family not supported by protocol
[1607371992] unbound[10523:0] fatal error: could not open ports

The requirement was first introduced [0] and only after the release
fixed in the upstream unit files [1]. The upstream fix is the same fix
we are applying to our unit configuration.

[0] https://github.com/NLnetLabs/unbound/commit/f6a527c25ad2e60e2dc129fff3605e6ec48c30f2
[1] https://github.com/NLnetLabs/unbound/pull/351/commits/c48f01445e22686a0cb83451ab055cb405940b58

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