bug in tzcode2001d.tar.gz's difftime.c

The difftime.c in tzcode2001d doesn't compile for me: it uses `sizeof T', where T is a type, but the C standard requires types to be parenthesized when they are the operands of sizeof. That was the only change in difftime.c that appeared in tzcode2001d, so I worked around the problem by reverting to the previous version of difftime.c.