Adding form key to cross sell page to prevent CSRF

To make sure post data for cross sell page is ignored when it fails form key check to avoid CSRF.

Part of update 1.9.4.5

This vulnerability affects the following application versions:

• Magento 1.9.2.3
• Magento 1.9.2.4
• Magento 1.9.3.0
• Magento 1.9.3.1
• Magento 1.9.3.2
• Magento 1.9.3.3
• Magento 1.9.3.4
• Magento 1.9.3.6
• Magento 1.9.3.7
• Magento 1.9.3.8
• Magento 1.9.3.9
• Magento 1.9.3.10
• Magento 1.9.4.0
• Magento 1.9.4.1
• Magento 1.9.4.2
• Magento 1.9.4.3
• Magento 1.9.4.4