Add maybe_unused to possibly unused argument

There are cases in this method where if CSRF protection is disabled,
this argument will not be used, and will trigger a compile error.  This
commit fixes the compile error.

Tested: Code compiles with CSRF disabled option set.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I6daa5193fa162c73c57991600058c198dc38a418
diff --git a/include/authentication.hpp b/include/authentication.hpp
index 18e7822..4897c0e 100644
--- a/include/authentication.hpp
+++ b/include/authentication.hpp
@@ -124,7 +124,7 @@
 
 #ifdef BMCWEB_ENABLE_COOKIE_AUTHENTICATION
 static std::shared_ptr<persistent_data::UserSession>
-    performCookieAuth(boost::beast::http::verb method,
+    performCookieAuth(boost::beast::http::verb method [[maybe_unused]],
                       const boost::beast::http::header<true>& reqHeader)
 {
     BMCWEB_LOG_DEBUG << "[AuthMiddleware] Cookie authentication";