x/tools/internal/robustio: build error on plan9 since CL 458200 due to missing definition for `getFileID`

[bcmills]

# golang.org/x/tools/internal/robustio
internal/robustio/robustio.go:65:58: undefined: getFileID
FAIL	golang.org/x/tools/internal/robustio [build failed]

A plan9 definition for that function appears to have been omitted in CL 458200 (attn @alandonovan @timothy-king).

[bcmills]

(CC @golang/plan9)

[adonovan]

see https://go.dev/cl/460855

[gopherbot]

Change https://go.dev/cl/460855 mentions this issue: internal/robustio: add Plan9 support to FileID