Fixed PHP Strict error WC_Product_Variation::get_price_html() should be compatible with WC_Product::get_price_html().
Fixed PHP strict warnings of WC_Product_Cat_List_Walker::*() should be compatible with that of Walker::*().
Declaration of WC_Product_Cat_Dropdown_Walker::start_el() should be compatible with Walker::start_el().
Declaration of WC_Product_Cat_Dropdown_Walker::start_el() should be compatible with Walker::start_el() - fixed parameter name.
Merge pull request #3420 from DeanMarkTaylor/bug/php-strict-warnings-01

Fixed PHP multiple strict errors and PHPDoc comments in WC_Product_Variation and WC_Product_Cat_List_Walker.