runtime: TestSpuriousWakeupsNeverHangSemasleep doesn't build on Plan 9

[0intro]

CL 135015 added TestSpuriousWakeupsNeverHangSemasleep. This test is failing on Plan 9 because syscall.SIGIO is not defined.

# runtime_test [runtime.test]
go/src/runtime/semasleep_test.go:74:23: undefined: syscall.SIGIO
FAIL	runtime [build failed]

See https://build.golang.org/log/a16a3aa8831cb7b424c37ccb0ce84a88ca27b5dc

[gopherbot]

Change https://golang.org/cl/135315 mentions this issue: runtime: don't build semasleep_test on Plan 9

[odeke-em]

Thanks for catching that @0intro! Interestingly though yesterday it built successfully https://go-review.googlesource.com/c/go/+/135015#message-01bdd240e139e50a967331d9c6095945f2ce702f

[0intro]

Unfortunately, the plan9 trybot has been disabled a while ago. The misc-compile-plan9 trybot only builds Go, without running the tests. However, the tests are only built when executed.
That's why this oversight hasn't been catched by the trybot.

[odeke-em]

Gotcha, thanks for the fix @0intro!