exo: build with -Wextra

exosphere/program/source/smc/secmon_smc_memory_access.cpp

@@ -69,6 +69,7 @@ namespace ams::secmon::smc {
 
     SmcResult SmcWriteAddress(SmcArguments &args) {
         /* NOTE: This smc was deprecated in Atmosphère 0.13.0. */
+        AMS_UNUSED(args);
         return SmcResult::NotImplemented;
     }