Source: Fix possible IWYU warnings in Windows generators

Source/cmGlobalVisualStudio9Generator.h

@@ -3,9 +3,13 @@
 #pragma once
 
 #include <memory>
+#include <string>
 
 #include "cmGlobalVisualStudio8Generator.h"
 
+class cmGlobalGeneratorFactory;
+class cmake;
+
 /** \class cmGlobalVisualStudio9Generator
  * \brief Write a Unix makefiles.
  *