Fix a gcc warning about copy

```
/home/jvoisin/dev/openmw/openmw/components/fontloader/fontloader.cpp:532:29: warning: loop variable '[key, value]' creates a copy from type 'std::pair<const int, int> const' [-Wrange-loop-construct]
            for (const auto [key, value] : additional)
                            ^
/home/jvoisin/dev/openmw/openmw/components/fontloader/fontloader.cpp:532:18: note: use reference type 'std::pair<const int, int> const &' to prevent copying
            for (const auto [key, value] : additional)
                 ^~~~~~~~~~~~~~~~~~~~~~~~~
                            &
1 warning generated.
```

components/fontloader/fontloader.cpp

@@ -529,7 +529,7 @@ namespace Gui
             additional.insert(std::make_pair(84, 0x2122)); // trademark sign
             additional.insert(std::make_pair(45, 0x2212)); // minus sign
 
-            for (const auto [key, value] : additional)
+            for (const auto& [key, value] : additional)
             {
                 if (key != i)
                     continue;