Nit - replace computing constant with that constant

`fontforge/parsettf.c :: readdate()` was computing the time difference between the 1904 epoch time ala Mac and the 1970 epoch time ala Unix. However this value is a constant and need not be recomputed each time. One could inspect the computed value once and make static, or derive the value from various references on the web.

This change replaces code and loop with initialization of a variable with the constant value.

Tested against three different ttf/ttc files and verified from the  "Element / Font Info... / Dates" displays before and after change.
Can we add a comment to that effect?