Fix conflicting sizer flags in wxWizard under wxOSX.

Don't use wxGROW and wxALIGN_CENTRE together in a wxBoxSizer, this doesn't
make sense as the latter is just ignored.

Also use wxEXPAND instead of wxGROW for consistency with the flags elsewhere
in the same file.

Closes #17063.

src/generic/wizard.cpp

@@ -411,7 +411,7 @@ void wxWizard::AddButtonRow(wxBoxSizer *mainColumn)
         mainColumn->Add(
             buttonRow,
             0, // Vertically unstretchable
-            wxGROW|wxALIGN_CENTRE
+            wxEXPAND
             );
     else
 #endif