syscall: fix plan9 build

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5532097
diff --git a/src/pkg/syscall/syscall_plan9.go b/src/pkg/syscall/syscall_plan9.go
index cd348f8..0bbb7ec 100644
--- a/src/pkg/syscall/syscall_plan9.go
+++ b/src/pkg/syscall/syscall_plan9.go
@@ -268,6 +268,11 @@
 	return Chdir(path)
 }
 
+type Timespec struct {
+	Sec  int32
+	Nsec int32
+}
+
 type Timeval struct {
 	Sec  int32
 	Usec int32