Add "#include <string>" so it stops complaining about "return NULL".

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@8832 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/tracks/check_manager.hpp

@@ -21,6 +21,7 @@
 #define HEADER_CHECK_MANAGER_HPP
 
 #include <vector>
+#include <string>
 
 class XMLNode;
 class CheckStructure;