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/cmInstallTargetGenerator.cxx

@@ -23,6 +23,7 @@
 #include "cmMessageType.h"
 #include "cmOutputConverter.h"
 #include "cmPolicies.h"
+#include "cmScriptGenerator.h"
 #include "cmStateTypes.h"
 #include "cmStringAlgorithms.h"
 #include "cmSystemTools.h"