Included 'mark.h' in the graphbuilder so that its method for removing the unused…

Included 'mark.h' in the graphbuilder so that its method for removing the unused param warning (casting to void) compiles on VS

include/yaml-cpp/contrib/graphbuilder.h

@@ -5,11 +5,11 @@
 #pragma once
 #endif
 
+#include "yaml-cpp/mark.h"
 #include <string>
 
 namespace YAML
 {
-  struct Mark;
   class Parser;
   
   // GraphBuilderInterface