Fix compiler error

Author: FranzBusch

Sources/FoundationEssentials/TimeCalculations.swift

@@ -148,8 +148,8 @@ extension Int64 {
         var years = remyears + (4 &* qCycles) + (100 &* cCycles) + (400 &* qcCycles)
 
         var months = 0
-        while Int64.daysInMonth(months) <= remdays {
-            remdays -= Int64.daysInMonth(months)
+        while daysInMonth(months) <= remdays {
+            remdays -= daysInMonth(months)
 
             // Unchecked because daysInMonth will crash if given a value greater than 12, so this
             // cannot exceed 12.