fix clang build
fix clang build

Summary: Clang doesn't like upcasting to incomplete types.

Test Plan: Build.

Reviewed By: aeckert@fb.com

Subscribers: net-systems@, mathieubaudet

FB internal diff: D2059151

Signature: t1:2059151:1431372362:219ce6bf1ac41f65d0e95b02e4579c682b094c66

neuhausler/fboss,biddyweb/fboss,sonoble/fboss,neuhausler/fboss,biddyweb/fboss,peterlei/fboss,peterlei/fboss,sonoble/fboss,raphaelamorim/fboss,peterlei/fboss,raphaelamorim/fboss,peterlei/fboss,sonoble/fboss,neuhausler/fboss,neuhausler/fboss,biddyweb/fboss,raphaelamorim/fboss