chore: adds licence comment to avoid warnings

[NelsonGaldeman]

Just adds the licence comment to avoid warnings like this when importing @Swapr on contracts built with newer compilers

Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.
--> /Users/dxdao/swapr-lp-zapping/node_modules/@swapr/core/contracts/interfaces/IDXswapPair.sol