Add alexrow to CODEOWNERS

[praveksharma]

#461 (and subsequently #530) attempted at add @alexrow to the CODEOWNERS file for this repository; this would give them write access to keep CROSS OIDs up to date. Since they haven't accepted the invitation to join the oqs-provider-codeowners team they don't have write access on this repository — GitHub complains about an invalid user being added to the CODEOWNERS file (see here).

To avoid holding up #530 from being merged, that PR does not update the CODEOWNERS file as required. @alexrow must be added to CODEOWNERS once possible.