add newer build constraint format

Go 1.17 introduced the new build constraint format[1] and `gofmt` now
complains if the new constraint format isn't present when the old
format is found.

[1] https://go.googlesource.com/proposal/+/master/design/draft-gobuild.md

Author: miabbott

activation/files_unix.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !windows
 // +build !windows
 
 // Package activation implements primitives for systemd socket activation.

examples/activation/activation.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build ignore
 // +build ignore
 
 // Activation example used by the activation unit tests.

examples/activation/httpserver/httpserver.go

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

examples/activation/listen.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build ignore
 // +build ignore
 
 // Activation example used by the activation unit tests.

examples/activation/udpconn.go

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build ignore
 // +build ignore
 
 // Activation example used by the activation unit tests.

util/util_cgo.go

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

util/util_stub.go

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