Bodge around the failing Coverity build in winshare.c.

The winegcc hack I use for my Coverity builds is currently using a
version of wincrypt.h that's missing a couple of constants I use.
Ensure they're defined by hand, but (just in case I defined them
_wrong_) also provide a command-line define so I can do that only in
the case of Coverity builds.


git-svn-id: svn://svn.tartarus.org/sgt/putty@10234 cda61777-01e9-0310-a592-d414129be87e