Define wxPG toolbar event handler only if library is compiled with toolbar classes.

Include wxPropertyGridManager::OnToolbarClick() code only if wxUSE_TOOLBAR is set to 1.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78214 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775