remove "selinux" build tag

The build tag was removed in go-selinux v1.8.0: opencontainers/selinux#132

Related: remove "apparmor" build tag: 0a9147f3aa

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

vendor/github.com/opencontainers/selinux/go-selinux/selinux_stub.go

@@ -1,4 +1,4 @@
-// +build !selinux !linux
+// +build !linux
 
 package selinux
 
@@ -146,3 +146,7 @@ func dupSecOpt(src string) ([]string, error) {
 func disableSecOpt() []string {
 	return []string{"disable"}
 }
+
+func getDefaultContextWithLevel(user, level, scon string) (string, error) {
+	return "", nil
+}