Allow linkage to libnss_files.so.2 on Linux

[sjackman]

libnss_nis.so.2 in glibc@2.13 has linkage to libnss_files.so.2.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Fix the error

$ brew linkage --test glibc@2.13
Unwanted system libraries:
  /lib/x86_64-linux-gnu/libnss_files.so.2

https://github.com/Homebrew/homebrew-core/runs/4676492184?check_suite_focus=true#step:7:67

[request-info]

Please provide a better issue/pull request title and/or description!

[BrewTestBot]

Review period will end on 2022-01-04 at 00:00:00 UTC.

[BrewTestBot]

Review period skipped due to critical label.

[iMichka]

Let's merge this, it's a minor change, just fixing an audit.