Fix strncat size argument for GCC9 (#8855)

Previously, the (LTO) linker complained:
```
In function ‘copy_object_names_to_clipboard’,
    inlined from ‘window_object_load_error_mouseup’ at ../src/openrct2-ui/windows/ObjectLoadError.cpp:450:43:
../src/openrct2-ui/windows/ObjectLoadError.cpp:358:16: error: ‘strncat’ specified bound 1 equals source length [-Werror=stringop-overflow=]
  358 |         strncat(buffer, PLATFORM_NEWLINE, line_sep_len);
      |                ^
```

src/openrct2-ui/windows/ObjectLoadError.cpp

@@ -355,7 +355,7 @@ static void copy_object_names_to_clipboard(rct_window* w)
         }
 
         strncat(buffer, _invalid_entries[i].name, nameLength);
-        strncat(buffer, PLATFORM_NEWLINE, line_sep_len);
+        strncat(buffer, PLATFORM_NEWLINE, buffer_len - strlen(buffer) - 1);
     }
 
     platform_place_string_on_clipboard(buffer);