test: sysfs: Fix readability-braces-around-statements

```
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../test/sysfs.cpp:46:28: error: statement should be inside braces [readability-braces-around-statements,-warnings-as-errors]
        if (dir == nullptr)
                           ^
                            {
```

Change-Id: Ib36d16ad9c7b716b397ba95fb13aa2d41ec1f195
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/test/sysfs.cpp b/test/sysfs.cpp
index 444d8b3..3385dee 100644
--- a/test/sysfs.cpp
+++ b/test/sysfs.cpp
@@ -44,7 +44,9 @@
         strncpy(buffer.data(), tmplt, buffer.size() - 1);
         char* dir = mkdtemp(buffer.data());
         if (dir == nullptr)
+        {
             throw std::system_error(errno, std::system_category());
+        }
 
         return FakeSysfsLed(fs::path(dir));
     }