Fix bug in vertical toolbar size calculation in wxMSW.

Correct the changes of the r76035 to avoid using separators when calculating
the fitting width of the vertical toolbars as this doesn't always work
correctly.

Closes #3788.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76080 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775