go/gcimporter15: require go1.6 for binary import/export (fix build)

The binary import/export functionality depends on go/constant from
go1.6 - make it only available if we build against go1.6. Eventually
this packet will be replaced by std lib go/importer functionality.

Change-Id: If96cf2e889daf1250152d7719afa64ad1ba8fb0e
Reviewed-on: https://go-review.googlesource.com/20716
Reviewed-by: Alan Donovan <adonovan@google.com>

go/gcimporter15/bexport.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build go1.6
+
 // Binary package export.
 // This file was derived from $GOROOT/src/cmd/compile/internal/gc/bexport.go;
 // see that file for specification of the format.

go/gcimporter15/bexport_test.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build go1.6
+
 package gcimporter_test
 
 import (