Fix typescript syntax error

[chandlerprall]

Supports elastic/kibana#20790 (comment)

[w33ble]

For reference, this was the error from the Kibana CI:

info  [kibana-oss] Transpiling sources with typescript compiler
18:42:24    │ debg  $ /var/lib/jenkins/workspace/elastic+kibana+pull-request+multijob-selenium/kibana/node_modules/typescript/bin/tsc --pretty true
18:42:31    │ debg  ../../node_modules/@elastic/eui/src/services/key_codes.d.ts:14:4 - error TS1036: Statements are not allowed in ambient contexts.
18:42:31    │ debg  
18:42:31    │ debg  14   };
18:42:31    │ debg        ~
18:42:31    │ debg  
18:42:31    │ debg  
18:42:31    │ERROR  failure
18:42:31    │ERROR  Error: Command failed: /var/lib/jenkins/workspace/elastic+kibana+pull-request+multijob-selenium/kibana/node_modules/typescript/bin/tsc --pretty true
18:42:31    │       
18:42:31    │       ../../node_modules/@elastic/eui/src/services/key_codes.d.ts:14:4 - error TS1036: Statements are not allowed in ambient contexts.
18:42:31    │       
18:42:31    │       14   };
18:42:31    │             ~
18:42:31    │       
18:42:31    │       
18:42:31    │       
18:42:31    │           at makeError (/var/lib/jenkins/workspace/elastic+kibana+pull-request+multijob-selenium/kibana/node_modules/execa/index.js:172:9)
18:42:31    │           at Promise.all.then.arr (/var/lib/jenkins/workspace/elastic+kibana+pull-request+multijob-selenium/kibana/node_modules/execa/index.js:277:16)
18:42:31    │           at <anonymous>
18:42:31    │           at process._tickCallback (internal/process/next_tick.js:188:7)

[w33ble]

I have no idea how typescript and its definitions work, but I'll approve this if it makes you feel better about the change.

[chandlerprall]

jenkins test this