Fix false positive on non enum classes (#34)

Author: jdecool

src/Rule/NoDuplicateEnumValueRule.php

@@ -4,6 +4,7 @@
 
 namespace Timeweb\PHPStan\Rule;
 
+use MyCLabs\Enum\Enum;
 use PhpParser\Node;
 use PhpParser\Node\Stmt\Class_;
 use PHPStan\Analyser\Scope;
@@ -32,6 +33,10 @@ public function processNode(Node $node, Scope $scope): array
             throw new ShouldNotHappenException();
         }
 
+        if (!$classReflection->isSubclassOf(Enum::class)) {
+            return [];
+        }
+
         $duplicatedKeysValue = $this->findDuplicatedKeys($node);
         if (empty($duplicatedKeysValue)) {
             return [];