libstrat: enable -Wextra, -Werror

This caught an embarrassingly large number of bugs.

libraries/libstratosphere/source/fssystem/fssystem_allocator_utility.cpp

@@ -29,6 +29,7 @@ namespace ams::fssystem {
         }
 
         void DefaultDeallocate(void *ptr, size_t size) {
+            AMS_UNUSED(size);
             std::free(ptr);
         }