Don't change header window foreground colour in wxGenericListCtrl.

As calling wxGenericListCtrl::SetBackgroundColour() doesn't change the header
window background, don't change its foreground in SetForegroundColour()
neither.

Closes #15741.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75325 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775