minor symfony#12801 Use HttpKernel Component (demyanovs)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes symfony#12801).

Discussion
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Use HttpKernel Component

We need to import Symfony\Component\HttpKernel before using it.

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Commits
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2cf09a9 Use HttpKernel Component

Author: javiereguiluz

create_framework/http_kernel_httpkernelinterface.rst

@@ -56,6 +56,7 @@ PHP; it implements ``HttpKernelInterface`` and wraps another
     // example.com/web/front.php
 
     // ...
+    use Symfony\Component\HttpKernel;
 
     $framework = new Simplex\Framework($dispatcher, $matcher, $controllerResolver, $argumentResolver);
     $framework = new HttpKernel\HttpCache\HttpCache(