cm_codecvt: Tell IWYU to keep cmConfigure.h

We no longer need it explicitly for `CM_OVERRIDE`, but we still need to
include it first everywhere.

Source/cm_codecvt.hxx

@@ -3,7 +3,7 @@
 #ifndef cm_codecvt_hxx
 #define cm_codecvt_hxx
 
-#include "cmConfigure.h"
+#include "cmConfigure.h" // IWYU pragma: keep
 
 #include <locale>
 #include <wchar.h>