Add missing include in util.cc

[ncfavier]

The clang build error that #5197 was trying to fix seems to be caused by a missing <array> header in util.cc (cc @andir #5075).

This is completely untested but should be obvious enough.

[happysalada]

I'm starting a local darwin build, I'll update with the results when it's done (my machine is quite old, this might take some time).

[ncfavier]

Test failure unrelated.

[happysalada]

Yeah I was having a look and it doesn't seem to be related to this.
What gives me hope is that the compilation went ahead until the end.

[happysalada]

The build is just finished and it worked like a charm!
great job on finding this fix!

❯ ./result/bin/nix --version
nix (Nix) 2.4pre20210908_7f0d177

if you want to make a PR on nixpkgs that includes this patch, I'm happy to review it and merge it.
we can also wait a couple of days for upstream if you want.

[ncfavier]

I'm fine with waiting. I expect this to be merged quickly anyway

[edolstra]

Thanks!