Make xfrm linux-only

The xfrm framework is linux-only. Only implement the respective types
for GOOS=linux to avoid dependencies to x/sys/unix on non-linux or
non-unix platforms. Provide dummy XfrmPolicy and XfrmState types for the
globally defined XfrmPolicy* and XfrmState* functions.

xfrm_unspecified.go

@@ -0,0 +1,7 @@
+//go:build !linux
+// +build !linux
+
+package netlink
+
+type XfrmPolicy struct{}
+type XfrmState struct{}