Include <wx/textctrl.h> from <wx/combobox.h> for compatibility.

Include <wx/textctrl.h> from <wx/combobox.h> for compatibility.

The wxComboBox header itself doesn't need <wx/textctrl.h> but there exists
code using wxComboBox with wxTextCtrl styles and events that doesn't include
<wx/textctrl.h> explicitly because it wasn't necessary in 2.8. So include it
from wx/combobox.h to avoid breaking this code after an upgrade to 2.9.

Closes #14132.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70997 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775