Fix `go vet` issues

Test: go vet ./...
Change-Id: Ifb936ccc5e2b5a2c3fcbbbcb54f680e2973ea1b3
diff --git a/genrule/genrule.go b/genrule/genrule.go
index 03d4ea6..8fedc60 100644
--- a/genrule/genrule.go
+++ b/genrule/genrule.go
@@ -142,7 +142,7 @@
 	if g, ok := ctx.Module().(*Module); ok {
 		if len(g.properties.Tools) > 0 {
 			ctx.AddFarVariationDependencies([]blueprint.Variation{
-				{"arch", ctx.Config().BuildOsVariant},
+				{Mutator: "arch", Variation: ctx.Config().BuildOsVariant},
 			}, hostToolDepTag, g.properties.Tools...)
 		}
 	}