typedef: make ssize_t get typedef without LIBSSH2_WIN32
typedef: make ssize_t get typedef without LIBSSH2_WIN32

The condition around the ssize_t typedef depended on both LIBSSH2_WIN32
*and* _MSC_VER being defined when it should be enough to depend on
_MSC_VER only. It also makes it nicer so libssh2-using code builds fine
without having custom defines.

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2