fix: refine feature checks for SELinux and SMACK support

[xtqqczze]

Resolve build failures on non-Linux platforms when selinux or smack features are enabled. The cfg attribute was missing target_os check, causing pre-commit hook failures.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/retry. tests/tail/retry is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/pipe-f is now passing!

[codspeed-hq]

Merging this PR will not alter performance

✅ 311 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing xtqqczze:mknod-selinux-smack (d366090) with main (5aade31)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[cakebaker]

Thanks!