build: typo from the cross-compile patchset that makes Windows build fail

By the way, the last few commits have touched the build system and will
most likely result in build failures unless you "make clean" and
./configure again. Sorry for the inconvenience.

(Patch by Adrien Nader)

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13946 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02