IWYU: Update for Debian 12 CI job

`include-what-you-use` diagnostics, in practice, are specific to
the environment's compiler and standard library.  Update includes
to satisfy IWYU for our CI job under Debian 12.

Source/cmDebuggerExceptionManager.h

@@ -5,7 +5,6 @@
 #include "cmConfigure.h" // IWYU pragma: keep
 
 #include <cstddef>
-#include <functional>
 #include <mutex>
 #include <string>
 #include <unordered_map>
@@ -14,7 +13,7 @@
 
 #include <cm3p/cppdap/protocol.h>
 
-#include "cmMessageType.h"
+#include "cmMessageType.h" // IWYU pragma: keep
 
 namespace dap {
 class Session;