[Simple] Weight header snippet missing include guards

[mbartling]

https://github.com/uTensor/utensor_cgen/blob/develop/utensor_cgen/backend/snippets/templates/containers/weight_header.hpp missing include guards similar to

https://github.com/uTensor/utensor_cgen/blob/develop/utensor_cgen/backend/snippets/templates/snippets/get_ctx.hpp

It's an easy fix and I'd normally get to it. For now, documenting