use require for buffer-layout

[0xCactus]

for fixing the following error

TSError: ⨯ Unable to compile TypeScript:

../public/solend-sdk/src/state/obligation.ts(3,31): error TS7016: Could not find a declaration file for module 'buffer-layout'. '/home/solend/app/node_modules/buffer-layout/lib/Layout.js' implicitly has an 'any' type.

  Try `npm i --save-dev @types/buffer-layout` if it exists or add a new declaration (.d.ts) file containing `declare module 'buffer-layout';`

adding declare module didn't help