Code Submition ( Improvement for mpn_mul_fft )

[Jeunder Yu]

> static void
> mpn_fft_fft_sqr (mp_ptr *Ap, mp_size_t K,
>                 mp_size_t omega, mp_size_t n, mp_ptr tp)
> {
>  mp_size_t node = 1, step = K >> 1;
>  /* top-down traversal, set node = 1 -> root for initial */
>
>  do
>    {
>      mp_limb_t cy;
> #if HAVE_NATIVE_mpn_addsub_n
>      ca = mpn_addsub_n (Ap[0], Ap[step], Ap[0], Ap[step], n + 1) & 1;

In above line, changes "ca = ..." to "cy = ..."

It is a slip of "copy and paste". The value of HAVE_NATIVE_mpn_addsub_n is 
false on my machine, so I am not aware of this mistake.