libstrat: enable -Wextra, -Werror

This caught an embarrassingly large number of bugs.

libraries/libstratosphere/source/fssystem/fssystem_hierarchical_sha256_storage.hpp

@@ -50,6 +50,7 @@ namespace ams::fssystem {
             }
 
             virtual Result SetSize(s64 size) override {
+                AMS_UNUSED(size);
                 return fs::ResultUnsupportedOperationInHierarchicalSha256StorageA();
             }
     };