Add 3 missing declarations for wxPoint2DInt non-member operator functions.

Three duplicate declarations were removed in r70493 instead of replacing them with the missing declarations.

Closes #10946.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76568 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775