api: fix build for go/build change

Fixes build.

Change-Id: I5235deed7b93157854acb79c15e2e99cc28d300a
Reviewed-on: https://go-review.googlesource.com/18456Reviewed-by: Russ Cox <rsc@golang.org>

api/go1.6.txt

@@ -178,8 +178,8 @@ pkg encoding/asn1, const TagUTCTime = 23
 pkg encoding/asn1, const TagUTCTime ideal-int
 pkg encoding/asn1, const TagUTF8String = 12
 pkg encoding/asn1, const TagUTF8String ideal-int
-pkg go/build, const AllowVendor = 8
-pkg go/build, const AllowVendor ImportMode
+pkg go/build, const IgnoreVendor = 8
+pkg go/build, const IgnoreVendor ImportMode
 pkg go/build, type Package struct, InvalidGoFiles []string
 pkg go/constant, func ToComplex(Value) Value
 pkg go/constant, func ToFloat(Value) Value