runtime: add a missing import
runtime: add a missing import

This adds an import of the runtime package to fix compilation
of the TestStopCPUProfilingWithProfilerOff function.

The gccgo compiler should never have accepted this.  The patch
for the comiler is https://golang.org/cl/116960043 .
The test is https://golang.org/cl/118000043 .

R=iant
CC=gofrontend-dev
https://golang.org/cl/110710043

anlhord/gofrontend,anlhord/gofrontend,golang/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,golang/gofrontend,anlhord/gofrontend,golang/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,golang/gofrontend