US167 17.03.1 Feature-test macro for non-member ssize() P1902

[wg21bot]

We forgot another feature test macro.

Proposed change:
Add a new entry to the table:
__cpp_lib_nonmember_signed_size | 201907L | <iterator>

[dietmarkuehl]

Reviewed Monday morning in Belfast in LWG small group session.

[brevzin]

SG10 consensus on the the new macro in D1902R1.

[mclow]

Resolved by the adoption of P1902

[jensmaurer]

Accepted.