avahi: Simplify pkg-config cross fix
avahi: Simplify pkg-config cross fix

Building `pkgsCross.aarch64-multiplatform.avahi` would fail in the past with:

    checking for pkg-config... no
    configure: error: pkg-config is required to install this program

To fix that, two independent workarounds were applied, each sufficient:

- 34e4d0fd98aa66a0f46a6eb96dabadbbd44294b9
- 65a5313de5d34d73e2930bd132fe476f57f1b60f

These days, it is more common to just add `pkg-config` to `depsBuildBuild`.

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