func fieldPrefix(fld []*ast.Field) string {
	prefix := ""
	for _, f := range fld {
		for _, n := range f.Names {
			// Ignore field names that don't have the prefix we're
			// looking for. It is common in C headers to have fields
			// named, say, _pad in an otherwise prefixed header.
			// If the struct has 3 fields tv_sec, tv_usec, _pad1, then we
			// still want to remove the tv_ prefix.