Support priv/unpriv image extraction

Optionally add back privileged extraction which can be enabled with
LUET_PRIVILEGED_EXTRACT=true

Signed-off-by: Ettore Di Giacinto <mudler@sabayon.org>

vendor/github.com/moby/buildkit/util/system/seccomp_nolinux.go

@@ -0,0 +1,7 @@
+// +build !linux,seccomp
+
+package system
+
+func SeccompSupported() bool {
+	return false
+}