Add some return types to fix deprecations

[javiereguiluz]

It fixes these two indirect deprecations in my Symfony apps:

• Method "Symfony\Component\HttpKernel\Bundle\Bundle::build()" might add "void" as a native return type declaration in the future.
• Method "Symfony\Component\DependencyInjection\Extension\ExtensionInterface::load()" might add "void" as a native return type declaration in the future.