Core/ConditionMgr: Add string include

Travis/Clang did not complain, GCC seems to do.

src/server/game/Conditions/ConditionMgr.h

@@ -23,6 +23,7 @@
 #include "Errors.h"
 #include <list>
 #include <map>
+#include <string>
 
 class Player;
 class Unit;