Move require to top level of the module
Move require to top level of the module

Conditionally importing does not seem to be required (constants only).

When running tests in parallel with Jest and a mysql2 connection we receive warnings as below

```
ReferenceError: You are trying to `import` a file after the Jest environment has been torn down.

      at new ResultSetHeader (node_modules/mysql2/lib/packets/resultset_header.js:39:32)
      at Query.resultsetHeader (node_modules/mysql2/lib/commands/query.js:102:16)
      at Query.execute (node_modules/mysql2/lib/commands/command.js:39:22)
      at Connection.handlePacket (node_modules/mysql2/lib/connection.js:417:32)
      at PacketParser.onPacket (node_modules/mysql2/lib/connection.js:75:12)
```

If the require statement is moved to the top of the file as per this commit the issue is resolved.
sidorares/node-mysql2,sidorares/node-mysql2,sidorares/node-mysql2,sidorares/node-mysql2