uk/plat: Mark MRD validation flags variable in macro as maybe unused

uk/plat: Mark MRD validation flags variable in macro as maybe unused

There may be unused warnings for this variable in builds that do not
have assertions enabled. Therefore, make it so that it does not generate
such warnings if assertions are disabled.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
GitHub-Closes: #1414