Add return type to DummyTestSplFileInfo::getRealPath()

Signed-off-by: Alexander M. Turek <me@derrabus.de>

tests/Differ/DummyTestSplFileInfo.php

@@ -16,7 +16,7 @@ namespace PhpCsFixer\Tests\Differ;
 
 final class DummyTestSplFileInfo extends \SplFileInfo
 {
-    public function getRealPath()
+    public function getRealPath(): string
     {
         return $this->getFilename();
     }