xds: Add xds dependency to the fallback test client (#5062)

diff --git a/interop/grpclb_fallback/client_linux.go b/interop/grpclb_fallback/client_linux.go
index c9b25a8..b5a16bf 100644
--- a/interop/grpclb_fallback/client_linux.go
+++ b/interop/grpclb_fallback/client_linux.go
@@ -35,6 +35,7 @@
 	"google.golang.org/grpc/credentials"
 	"google.golang.org/grpc/credentials/alts"
 	"google.golang.org/grpc/credentials/google"
+	_ "google.golang.org/grpc/xds/googledirectpath"
 
 	testgrpc "google.golang.org/grpc/interop/grpc_testing"
 	testpb "google.golang.org/grpc/interop/grpc_testing"