strat: build sysmodules with -Wextra/-Werror

stratosphere/ams_mitm/source/bpc_mitm/bpcmitm_module.cpp

@@ -56,7 +56,7 @@ namespace ams::mitm::bpc {
 
     }
 
-    void MitmModule::ThreadFunction(void *arg) {
+    void MitmModule::ThreadFunction(void *) {
         /* Wait until initialization is complete. */
         mitm::WaitInitialized();