refactor(IWYU): add "private" pragma to more generated headers (#26706)

"export" only prevents IWYU from adding these headers if the headers
that export them are included, while "private" ensures that IWYU never
adds these headers.

src/nvim/option_defs.h

@@ -131,5 +131,5 @@ typedef enum {
 } OptReqScope;
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
-# include "options_enum.generated.h"  // IWYU pragma: export
+# include "options_enum.generated.h"
 #endif