You should inject class `Magento\Store\Model\System\Store` in constructor and declare a global variable.

E.g: 

    public function __construct(
       //...,
       Magento\Store\Model\System\Store $systemStore
    ) {
       //...
      $this->_systemStore = $systemStore;
    }

     $fieldset->addField(
         'store_ids',
         'multiselect',
         [
             'name'     => 'store_ids[]',
             'label'    => __('Store Views'),
             'title'    => __('Store Views'),
             'required' => true,
             'values'   => $this->_systemStore->getStoreValuesForForm(false, true),
         ]
      );