fixes "System.Reflection.AmbiguousMatchException"

```
System.Reflection.AmbiguousMatchException: Multiple custom attributes of the same type found.
   at System.Attribute.GetCustomAttribute(Assembly element, Type attributeType, Boolean inherit)
   at System.Reflection.CustomAttributeExtensions.GetCustomAttribute[T](Assembly element)
   at Pchp.Core.Reflection.PhpStackFrame.get_TypeName()
   at Pchp.Core.Reflection.PhpStackTrace.FrameLine.ToUserFrame()
```

src/Peachpie.Runtime/Reflection/PhpStackTrace.cs

@@ -341,7 +341,8 @@ public string TypeName
 
                     if (tinfo.IsPublic && tinfo.IsAbstract) // => public static
                     {
-                        if (tinfo.Assembly.IsDefined(typeof(PhpExtensionAttribute)))
+                        var extensionAttrs = Attribute.GetCustomAttributes(tinfo.Assembly, typeof(PhpExtensionAttribute));
+                        if (extensionAttrs != null && extensionAttrs.Length != 0)
                         {
                             // library function
                             return null;