runtime: define netpollinited on Plan 9

Since CL 2750, the build is broken on Plan 9,
because a new function netpollinited was added
and called from findrunnable in proc1.go.
However, netpoll is not implemented on Plan 9.
Thus, we define netpollinited in netpoll_stub.go.

Fixes #9590

Change-Id: I0895607b86cbc7e94c1bfb2def2b1a368a8efbe6
Reviewed-on: https://go-review.googlesource.com/2759Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/netpoll_stub.go

@@ -13,3 +13,7 @@ func netpoll(block bool) (gp *g) {
 	// integrated network poller.
 	return
 }
+
+func netpollinited() bool {
+	return false
+}