Mention wxDD_DIR_MUST_EXIST support in wxGTK in the change log.

Document the change of r75997, see #16002.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76000 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775