Initialize time zone information before using it in wxGetTimeZone().

Initialize time zone information before using it in wxGetTimeZone().

We must call _tzset() before calling _get_timezone() as while this is normally
done implicitly by the other time functions, it might not have been done yet
if create a wxDateTime::TimeZone before calling any of them.

Closes #12700.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66241 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775