Add return type to TwigFormPass::process() (#518)

This fixes the warning:

Method
"Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface::process()"
might add "void" as a native return type declaration in the future.
Do the same in implementation
"FM\ElfinderBundle\DependencyInjection\Compiler\TwigFormPass"
now to avoid errors or add an explicit @return annotation to suppress this message.

src/DependencyInjection/Compiler/TwigFormPass.php

@@ -7,7 +7,7 @@
 
 class TwigFormPass implements CompilerPassInterface
 {
-    public function process(ContainerBuilder $container)
+    public function process(ContainerBuilder $container): void
     {
         if (!$container->hasParameter('twig.form.resources')) {
             return;