ocamlPackages.luv: backport patch; fix clang build
clang build fails with an incompatible pointer type error. Backport a
patch from 0.5.14 which fixes this. Can not just update to 0.5.14 as it
contains breaking changes and breaks most of luv's consumers.
https://github.com/aantron/luv/commit/ad7f953fccb8732fe4eb9018556e8d4f82abf8f2