MessageFormat: Use int64 instead of off_t

They are effectively the same thing, but int64 is more generic.

I used an off_t because that is what Tracker passed in, yet, this could be
used in other places where an off_t doesn't make as much sense.

headers/os/locale/MessageFormat.h

@@ -28,7 +28,7 @@ public:
 
 			status_t			InitCheck();
 
-			status_t			Format(BString& buffer, const off_t arg) const;
+			status_t			Format(BString& buffer, const int64 arg) const;
 
 private:
 			status_t			_Initialize(const icu::UnicodeString&);