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/cmGeneratorTarget.h

@@ -17,6 +17,7 @@
 #include <cm/optional>
 
 #include "cmAlgorithms.h"
+#include "cmComputeLinkInformation.h"
 #include "cmLinkItem.h"
 #include "cmListFileCache.h"
 #include "cmPolicies.h"
@@ -24,7 +25,6 @@
 #include "cmValue.h"
 
 enum class cmBuildStep;
-class cmComputeLinkInformation;
 class cmCustomCommand;
 class cmFileSet;
 class cmGlobalGenerator;