test: physical: Fix cppcoreguidelines-explicit-virtual-functions

```
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../test/physical.cpp:51:13: error: prefer using 'override' or (rarely) 'final' instead of 'virtual' [cppcoreguidelines-explicit-virtual-functions,modernize-use-override,-warnings-as-errors]
    virtual ~MockLed()
    ~~~~~~~~^
                       override
```

Change-Id: If8fa33778605f485991723c24c3568d1c1af1cbb
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
1 file changed