QStorageInfo: Fix warning about unused variable

Happens on non-Linux, non-macOS Unix systems (got it on FreeBSD).

Change-Id: Ie67d35dff21147e99ad9fffd14acc7308b5ff17e
Reviewed-by: Jake Petroules <jake.petroules@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

src/corelib/io/qstorageinfo_unix.cpp

@@ -503,6 +503,8 @@ static QByteArray extractSubvolume(const QStorageIterator &it)
         // if we didn't find the subvolume name, return the subvolume ID
         return id;
     }
+#else
+    Q_UNUSED(it);
 #endif
     return QByteArray();
 }