unix: keep generated types compilable with current cgo

[librepower]

I hit this while regenerating ztypes for a platform that is not in the tree
yet, but it is not specific to that: any regeneration with a current
toolchain runs into it.

Where struct fp_extended_precision is not declared, cgo -godefs now
renders FpExtendedPrecision as _cgopackage.Incomplete, and the result
does not compile. The committed files for those platforms carry an empty
struct, so this keeps mkpost producing what is already there.

freebsd/arm is unaffected: the C type is declared there, so cgo never emits
Incomplete for it.

Happy to widen the scope or drop it if you would rather handle this some
other way.

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[librepower]

I signed it!

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Patch Set 1: Commit-Queue+1

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