chcon, runcon: Use compile_error! to prevent incorrectly distributing them

[oech3]

Distributors do not want to publish them if Makefile included a bug by some reason.

[xtqqczze]

I don't want compile errors blocking clippy and pre-commit hook: #11019

[oech3]

While clippy does not appear in productions, stub appears in productions.
Production should have high-priority.

[oech3]

We just need something like

#[cfg(*)]
chcon = { optional = true, version = "0.8.0", package = "uu_chcon", path = "src/uu/chcon" }