Fix SMPP timestamp decoding when the current date/time's offset to UTC is
different from the timestamp's offset to UTC.

The fix for bug 6712 ignored the fact that daylight savings will affect the
offset from UTC--and whether daylight savings is in effect today might be
different than whether it was in effect in the timestamp we're decoding.

Bug: 10672
Change-Id: Ie513c361cff41007f2328cd9ca9ca4800ffef500
Reviewed-on: https://code.wireshark.org/review/5245
Petri-Dish: Michael Mann <mmann78@netscape.net>
Petri-Dish: Jeff Morriss <jeff.morriss.ws@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>