Fix c++20 issue

c++20 seems to be enforcing that pragma once is the first directive in a
header file.  This seems reasonable to have, and I didn't look much
further into the standard for the specific rule.

Tested:
Patched another nlohmann::json issue, then openbmc-build-scripts now
builds with this change.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I5538f8bb0c97b0602d3b9d133e4c85b0a5893773
diff --git a/redfish-core/lib/redfish_util.hpp b/redfish-core/lib/redfish_util.hpp
index 60626a5..7345f01 100644
--- a/redfish-core/lib/redfish_util.hpp
+++ b/redfish-core/lib/redfish_util.hpp
@@ -13,8 +13,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 */
-#ifndef BMCWEB_ENABLE_REDFISH_ONE_CHASSIS
 #pragma once
+#ifndef BMCWEB_ENABLE_REDFISH_ONE_CHASSIS
 
 namespace redfish
 {