contrib: add minimal errno.h target (for WinCE)

contrib: add minimal errno.h target (for WinCE)

WinCE lacks errno support and thus doesn't have an errno.h header.
Since it is used like everywhere, contribs and VLC, we add a .errno
target in the contribs, that fetches a minimal errno.h that allows
everything to compile smoothly.

This is how it was done before too.