Reformat code according to 'gotip' rules.

In practices adds annotations in the new syntax:
```
+//go:build !linux
 // +build !linux
```

Fixes failing gotip PASSES='fmt' check:
https://travis-ci.com/github/etcd-io/etcd/jobs/486453806

pkg/netutil/routes.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !linux
 // +build !linux
 
 package netutil

pkg/netutil/routes_linux.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build linux
 // +build linux
 
 package netutil

pkg/netutil/routes_linux_test.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build linux
 // +build linux
 
 package netutil