test: add missing import for zerodivide

R=rsc
CC=golang-dev, r
https://golang.org/cl/1706043
diff --git a/test/zerodivide.go b/test/zerodivide.go
index ce0fc82..9d35b39 100644
--- a/test/zerodivide.go
+++ b/test/zerodivide.go
@@ -10,6 +10,7 @@
 	"fmt"
 	"math"
 	"strings"
+	"syscall"
 )
 
 type Error interface {