Fixed mpn_sumdiff_n reference in fft.h.
Fixed mpn_sumdiff_n reference in fft.h.

fredrik-johansson/flint2,dsroche/flint2,wbhart/flint2,dsroche/flint2,dsroche/flint2,dsroche/flint2,jpflori/flint2,jpflori/flint2,wbhart/flint2,jpflori/flint2,jpflori/flint2,fredrik-johansson/flint2,fredrik-johansson/flint2,wbhart/flint2