mhparse.c: Cast ptrdiff_t from subtraction to long.

Cygwin's 32-bit doesn't like `%ld' for the `p - q' difference between
two pointers as the value is an int, not a long.  printf(3)'s `t' length
modifier, for ptrdiff_t, can't be used as that's C99, and we're C90.
Cast the difference to a long explicitly instead.