add back import

Author: fchapoton

src/sage/rings/polynomial/polynomial_zmod_flint.pyx

@@ -40,6 +40,7 @@ AUTHORS:
 # ****************************************************************************
 
 from sage.libs.ntl.ntl_lzz_pX import ntl_zz_pX
+from sage.structure.factorization import Factorization  # removing breaks build
 from sage.structure.element cimport parent
 from sage.structure.element import coerce_binop
 from sage.rings.polynomial.polynomial_integer_dense_flint cimport Polynomial_integer_dense_flint