[numerics.c.ckdint] Add cross-reference to [basic.fundamental]

[eisenwave]

This fixes two problems:

• "cv-unqualified" is redundant here; the set of signed or unsigned integer types does not include any cv-qualified types.
• Usually, we cross reference [basic.fundamental] when using the term "signed or unsigned integer type", and there is no obvious reason why we shouldn't also do this here, for consistency.

[jensmaurer]

Adding the cross-reference is fine.

We have at least one other instance of "cv-unqualified signed/unsigned integer type"; it would be better to have a separate pull request that removes "cv-unqualified" from them all.