OpenBSD: update platform file

This is basically what they patch it to be downstream, so follow that.

See http://www.openbsd.org/cgi-bin/cvsweb/ports/devel/cmake/patches/patch-Modules_Platform_OpenBSD_cmake?rev=1.11;content-type=text%2Fplain
Merge topic 'OpenBSD-platform' into next

5822f299 OpenBSD: update platform file