finatra: Fix Inject App Doc Generation on 2.12
finatra: Fix Inject App Doc Generation on 2.12

Problem / Solution

Finatra build for 2.12 fails due to a Scala 2.12 compiler issue
(SI-10134). This adds a workaround when compiling the docs for
inject-app which allows it to succeed.

RB_ID=908920

twitter/finatra,twitter/finatra,twitter/finatra