libnetwork/slirp4netns: split out const

The cross tests needs at least one empty file in the package in order to
work and we need to linux build tag om slirp4netns as it depends on linux
only code.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

common/libnetwork/slirp4netns/const.go

@@ -0,0 +1,12 @@
+package slirp4netns
+
+const (
+	ipv6ConfDefaultAcceptDadSysctl = "/proc/sys/net/ipv6/conf/default/accept_dad"
+	BinaryName                     = "slirp4netns"
+
+	// defaultMTU the default MTU override
+	defaultMTU = 65520
+
+	// default slirp4ns subnet
+	defaultSubnet = "10.0.2.0/24"
+)

common/libnetwork/slirp4netns/slirp4netns.go

@@ -1,3 +1,6 @@
+//go:build linux
+// +build linux
+
 package slirp4netns
 
 import (
@@ -92,17 +95,6 @@ func (w *logrusDebugWriter) Write(p []byte) (int, error) {
 	return len(p), nil
 }
 
-const (
-	ipv6ConfDefaultAcceptDadSysctl = "/proc/sys/net/ipv6/conf/default/accept_dad"
-	BinaryName                     = "slirp4netns"
-
-	// defaultMTU the default MTU override
-	defaultMTU = 65520
-
-	// default slirp4ns subnet
-	defaultSubnet = "10.0.2.0/24"
-)
-
 func checkSlirpFlags(path string) (*slirpFeatures, error) {
 	cmd := exec.Command(path, "--help")
 	out, err := cmd.CombinedOutput()