Auto merge of #1206 - semarie:mincore, r=gnzlbg

mincore has been removed from upcoming OpenBSD 6.5

remove test from CI on OpenBSD for "mincore" function.

it has been removed from upcoming 6.5:
- https://marc.info/?l=openbsd-cvs&m=154723400730318
- https://github.com/openbsd/src/commit/54e4f6b9a1dc183e1dc7c4a722c2646aa309246a

I am only removing the test from CI, not touching exported function from Rust libc to avoid a breaking change.