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/cmGeneratorExpressionParser.h

@@ -7,10 +7,9 @@
 #include <memory>
 #include <vector>
 
+#include "cmGeneratorExpressionEvaluator.h"
 #include "cmGeneratorExpressionLexer.h"
 
-struct cmGeneratorExpressionEvaluator;
-
 struct cmGeneratorExpressionParser
 {
   cmGeneratorExpressionParser(std::vector<cmGeneratorExpressionToken> tokens);