Fix bug in vertical toolbar size calculation in wxMSW.

Correct the changes of the r76034 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/branches/WX_3_0_BRANCH@76079 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775