Avoid -Wunused-result in the except sample

MinGW gcc 12 started giving this (useful) warning in STL build because
its std::vector::operator[] is declared with nodiscard attribute, but we
really don't want to use the result here, so suppress it.

samples/except/except.cpp

@@ -564,7 +564,7 @@ void MyFrame::OnShowAssert(wxCommandEvent& WXUNUSED(event))
 {
     // provoke an assert from wxArrayString
     wxArrayString arr;
-    arr[0];
+    wxUnusedVar(arr[0]);
 }
 
 #if wxUSE_THREADS