synaptics: Use GDate getters to retrieve the DMY values

As per commit 201b5a96 we use g_date_copy() to copy the date, however the GLib implementation is wrong as it doesn't preserve the original format of the date (whether is using julian days or dmy), and the synaptics driver access to the dmy values directly, without using the getter that would recompute the proper values. Causing a read error of unset values.

So, to avoid this, just use the g_date_get_* getters to retrieve the day month and year for for defining the print enroll id.