Remove compiler warning.

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@6636 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/challenges/challenge.cpp

@@ -149,6 +149,7 @@ void Challenge::addUnlockKartReward(const std::string &internal_name,
     }
     catch (std::exception& e)
     {
+        (void)e;   // avoid compiler warning
         throw std::runtime_error(StringUtils::insertValues("Challenge refers to unknown kart <%s>", internal_name.c_str()));
     }