libstrat: enable -Wextra, -Werror

This caught an embarrassingly large number of bugs.

libraries/libstratosphere/source/fssystem/fssystem_aes_xts_storage.cpp

@@ -25,6 +25,7 @@ namespace ams::fssystem {
         AMS_ASSERT(key_size == KeySize);
         AMS_ASSERT(iv_size  == IvSize);
         AMS_ASSERT(util::IsAligned(this->block_size, AesBlockSize));
+        AMS_UNUSED(key_size, iv_size);
 
         std::memcpy(this->key[0], key1, KeySize);
         std::memcpy(this->key[1], key2, KeySize);