Fix a TypeScript test

[ArtskydJ]

When I ran npm run type-test, I got a few error messages. One of the messages was:

Cannot find module 'mysql2' or its corresponding type declarations.

This PR fixes that error.

There are a few more typescript errors, but I don't know the right way to fix those.

npm run type-test
> mysql2@3.0.1 type-test
> node ./node_modules/typescript/bin/tsc -p tests.json && mocha typings/test --timeout 10000

index.d.ts:86:18 - error TS2320: Interface 'PoolConnection' cannot simultaneously extend types 'PoolConnection' and 'Connection'.
  Named property 'execute' of types 'PoolConnection' and 'Connection' are not identical.

86 export interface PoolConnection extends mysql.PoolConnection, Connection {
                    ~~~~~~~~~~~~~~

index.d.ts:86:18 - error TS2320: Interface 'PoolConnection' cannot simultaneously extend types 'PoolConnection' and 'Connection'.
  Named property 'unprepare' of types 'PoolConnection' and 'Connection' are not identical.

86 export interface PoolConnection extends mysql.PoolConnection, Connection {
                    ~~~~~~~~~~~~~~

Found 2 errors in the same file, starting at: index.d.ts:86