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

```
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../test/sysfs.cpp:54:5: error: annotate this function with 'override' or (rarely) 'final' [cppcoreguidelines-explicit-virtual-functions,modernize-use-override,-warnings-as-errors]
    ~FakeSysfsLed()
    ^
                    override
```

Change-Id: I5461d3aee9c22080d5b88fda6084e0a7419e369e
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/test/sysfs.cpp b/test/sysfs.cpp
index 3385dee..cc19fc1 100644
--- a/test/sysfs.cpp
+++ b/test/sysfs.cpp
@@ -51,7 +51,7 @@
         return FakeSysfsLed(fs::path(dir));
     }
 
-    ~FakeSysfsLed()
+    ~FakeSysfsLed() override
     {
         fs::remove_all(root);
     }