which is an organosilane of the formula X ,,SiY or an organosiloxane having the average formula X Y SlO wherein each X is an unsubstituted or inertly substituted alkyl or aryl radical or a hydrogen atom, a is 3 or 4, in has a value in the range from 1 to 3 inclusive, and b has maximally the value of m and at least such a value that in each molecule there are three substituents Y, and Y is a Si-N-l