build: don't set add-hosts option if empty

This looks like an oversight, all of the other options have similar
checks. This can interfere with generated provenance where "add-hosts"
will be marked as an argument to the build, even though it's not
actually being utilized.

Signed-off-by: Justin Chadwell <me@jedevc.com>

build/build.go

@@ -607,7 +607,9 @@ func toSolveOpt(ctx context.Context, node builder.Node, multiDriver bool, opt Op
 	if err != nil {
 		return nil, nil, err
 	}
-	so.FrontendAttrs["add-hosts"] = extraHosts
+	if len(extraHosts) > 0 {
+		so.FrontendAttrs["add-hosts"] = extraHosts
+	}
 
 	// setup shm size
 	if opt.ShmSize.Value() > 0 {