Winbuild: remove obsolete pixman dependency

It was an internal depencency of libgnomeui, which
is no longer used for GnuCash

I found one more reference to libgnomeui in the
crosbuild_webkit.sh script. I have removed it there as well.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@22813 57a11ea4-9604-0410-9ed3-97b8803252fd