oc-calendar: Initialise NSCalendarDate with a SYSTEMTIME struct

The method computes the date of a SYSTEMTIME structure, in which the day
within the month is given by the Nth occurrence of a weekday (see
[MS-OXOCAL] 2.2.1.39).

OpenChange/NSDate+MAPIStore.h

@@ -37,6 +37,8 @@
 
 - (BOOL) isNever; /* occurs on 4500-12-31 */
 
++ (NSCalendarDate *) dateFromSystemTime: (struct SYSTEMTIME) date
+                            andRuleYear: (uint16_t) rYear;
 @end
 
 NSComparisonResult NSDateCompare (id date1, id date2, void *);