[styles] Fix constraint on withStyles P parameter to allow StyledComponentProps

[pelotom]

The signature of withStyles wasn't quite right, because it disallowed using props that extend StyledComponentProps, e.g.

interface FooProps extends StyledComponentProps<'x' | 'y'> {
  a: number
  b: boolean
}
  
const ListItemContent = withStyles({ x: {}, y: {} })<FooProps>(props => <div />);