libstrat: enable -Wextra, -Werror

This caught an embarrassingly large number of bugs.

libraries/libstratosphere/source/fssystem/fssystem_hierarchical_sha256_storage.cpp

@@ -38,6 +38,7 @@ namespace ams::fssystem {
         AMS_ASSERT(layer_count == LayerCount);
         AMS_ASSERT(util::IsPowerOfTwo(htbs));
         AMS_ASSERT(hash_buf != nullptr);
+        AMS_UNUSED(layer_count);
 
         /* Set size tracking members. */
         this->hash_target_block_size = htbs;