Initialize wxDateTime::Tm::yday in the ctor.

Initialize wxDateTime::Tm::yday in the ctor.

Set Tm::yday to a fixed value in the ctor as well instead of not initializing
it at all.

This bug actually didn't have any visible consequences as yday is currently a
private member of struct tm and didn't seem to be used anywhere but it at
least avoids MinGW 4.x warnings about possibly uninitialized variable and
might become important in the future if we allow accessing this field.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65648 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775