nameser.h on OpenBSD lives in the standard location.

So no need to special-case OpenBSD in src/luv_portability.h for the standard location of nameser.h.

On the other hand, this will need to be revisited as OpenBSD's nameser.h which is installed doesn't have a definition for ns_t.
So it should probably pull nameser.h in it from ares, although this may be an issue if luv_portability.h is installed (is this true?) and ares' nameser.h isn't around.

For now, I think this diff should be merged though.