Fix: "invalid conversion from ‘int" when compiling version 1.0.5 (microhttpd 0.9.71)

[DGdev91]

Had issue compiling on ArchLinux, i currently have microhttpd 0.9.71. Seems like i'm not the only one, there's an open issue complaining for the same problem #2685

Using the MHD_Result enum instead of "int" solved the problem (still had to make a static cast for MHD_digest_auth_check, which returns an int)

[DGdev91]

The pach was intended for xmr-stak-rx, i made a pull request for the right branch. https://github.com/fireice-uk/xmr-stak/pull/2689

But actually the code looks the same, probably is useful on the main branch too