Fixed warnings with Visual studio

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@8664 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/states_screens/race_gui_base.hpp

@@ -29,7 +29,7 @@
 #include <SColor.h>
 namespace irr
 {
-    namespace video { class ITexture; class S3DVertex; }
+    namespace video { class ITexture; struct S3DVertex; }
 }
 using namespace irr;
 

src/tracks/quad.hpp

@@ -25,7 +25,7 @@
 
 namespace irr
 {
-    namespace video { class S3DVertex; }
+    namespace video { struct S3DVertex; }
 }
 using namespace irr;