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 http://codereview.appspot.com/116960043 .
The test is https://codereview.appspot.com/118000043 .

From-SVN: r212870

libgo/go/runtime/runtime_test.go

@@ -9,7 +9,7 @@ import (
 	// "io/ioutil"
 	// "os"
 	// "os/exec"
-	// . "runtime"
+	. "runtime"
 	"runtime/debug"
 	// "strconv"
 	// "strings"