Change printf to fprintf (as other err prints in same file) so it compiles.

Otherwise it halted with g++ 4.6 with the following test:
addons/addons_manager.cpp: In member function ‘void* AddonsManager::downloadIcons()’:
addons/addons_manager.cpp:157:41: error: cannot pass objects of non-trivially-copyable type ‘const string’ through ‘...’
addons/addons_manager.cpp:157:41: warning: format ‘%s’ expects argument of type ‘char*’, but argument 2 has type ‘const string’ [-Wformat]


git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@8622 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/addons/addons_manager.cpp

@@ -153,8 +153,9 @@ void *AddonsManager::downloadIcons()
             if(icon=="")
             {
                 if(UserConfigParams::logAddons())
-                    printf("[addons] No icon or image specified for '%s'.\n",
-                           addon.getId());
+                    fprintf(stderr,
+                            "[addons] No icon or image specified for '%s'.\n",
+                            addon.getId().c_str());
                 continue;
             }
             std::string save        = "icons/"+icon;