libnetwork: add missing stub for getInitializers()

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

libnetwork/drivers_unsupported.go

@@ -0,0 +1,8 @@
+//go:build !freebsd && !linux && !windows
+// +build !freebsd,!linux,!windows
+
+package libnetwork
+
+func getInitializers() []initializer {
+	return nil
+}