Add string include so compile completes.

Error was "string in namespace std does not name a type".


git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@10734 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/karts/kart_gfx.hpp

@@ -23,6 +23,7 @@
  *  This class implements all particle effects for a kart. */
 
 #include <vector>
+#include <string>
 
 class Kart;
 class ParticleEmitter;