Fix -Wformat warning in vtkDebugLeaks

Common/Core/vtkDebugLeaks.cxx

@@ -99,7 +99,7 @@ void vtkDebugLeaksHashTable::PrintTable(std::string& os)
     if (iter->second > 0 && !vtkDebugLeaksIgnoreClassesCheck(iter->first))
     {
       char tmp[256];
-      snprintf(tmp, 256, "\" has %i %s still around.\n", iter->second,
+      snprintf(tmp, 256, "\" has %u %s still around.\n", iter->second,
         (iter->second == 1) ? "instance" : "instances");
       os += "Class \"";
       os += iter->first;