osrm-backend branch 5.21 doesn't exist anymore

[blakeflei]

Build fails because branch 5.21 of osrm-backend isn't available anymore.

There are no releases for 5.21, but there is a release for 5.24 that works (5.23 fails for a similar reason to Issue 5741).

[blakeflei]

Submitted PR #64

[Zebreu]

Thanks! I have another branch where the very latest releases should work with a recent os, I just haven't spent time for it so 5.24 should work for a while.