[mdspan.layout.right.obs] Add missing constexpr to required_span_size.

[1uc]

The missing constexpr looks like a typo, because it's present in the overview; and every other method is constexpr.

[jwakely]

The constexpr was missing in the incoming paper, P0009R18, but I agree that it looks like an editorial mistake.

[jensmaurer]

@mhoemmen , any comments here?

[mhoemmen]

@jensmaurer Thanks for the heads-up! Thanks @1uc for pointing this out!

I agree that this must have been an editorial mistake. It was certainly the intention for required_span_size() to be marked constexpr in layout_right::mapping. It's already marked constexpr in the other two C++23 layout mappings (layout_left::mapping and layout_stride::mapping).