lib9: enable on Plan 9

This change depends on CL 57170052.

LGTM=rsc
R=rsc, bradfitz
CC=golang-codereviews
https://golang.org/cl/60840049
diff --git a/src/lib9/nulldir.c b/src/lib9/nulldir.c
index aa1a123..2157ff3 100644
--- a/src/lib9/nulldir.c
+++ b/src/lib9/nulldir.c
@@ -1,3 +1,5 @@
+// +build !plan9
+
 /*
 Inferno lib9/nulldir.c
 http://code.google.com/p/inferno-os/source/browse/lib9/nulldir.c