It is pretty wonky, it wants to return the default error handler. So you have to pass a pointer to a variable. Like this (untested):
xmlGenericErrorFunc handler;
initGenericErrorDefaultFunc(&handler);
If I understand your intentions properly, this is not the function you actually want to use to suppress errors. Use xmlSetGenericErrorFunc() instead. You can use initGenericErrorDefaultFunc() to restore it again. Pass NULL.