Fix FileSystem.cpp on Android to not use the removed string + char operator.

Source/Urho3D/IO/FileSystem.cpp

@@ -636,7 +636,7 @@ bool FileSystem::DirExists(const String& pathName) const
     if (URHO3D_IS_ASSET(fixedName))
     {
         // Split the pathname into two components: the longest parent directory path and the last name component
-        String assetPath(URHO3D_ASSET((fixedName + '/')));
+        String assetPath(URHO3D_ASSET((fixedName + "/")));
         String parentPath;
         unsigned pos = assetPath.FindLast('/', assetPath.Length() - 2);
         if (pos != String::NPOS)