Added missing className property to CardProps (#10823)

The className property for CardPros is missing, so using it like
<Card className={this.myClass}>...</Card>
is not possible.

material-ui/material-ui.d.ts

@@ -759,6 +759,7 @@ declare namespace __MaterialUI {
     namespace Card {
 
         interface CardProps extends React.Props<Card> {
+            className?: string;
             actAsExpander?: boolean;
             containerStyle?: React.CSSProperties;
             expandable?: boolean;