libstrat: enable -Wextra, -Werror

This caught an embarrassingly large number of bugs.

libraries/libstratosphere/source/lm/srv/lm_flush_thread.cpp

@@ -44,6 +44,7 @@ namespace ams::lm::srv {
         }
 
         void DeallocateForFs(void *ptr, size_t size) {
+            AMS_UNUSED(size);
             return lmem::FreeToExpHeap(g_fs_heap_handle, ptr);
         }