Update CMakeLists.txt snippet so that example compiles

Summary: Previous to this the example didn't work out of the box, it seems some cmake config changed between when this was written and now.

Author: Dan Zimmerman <daniel.zimmerman@me.com>

Reviewers: modocache, steveire

Reviewed By: steveire

Subscribers: smeenai, steveire, cfe-commits

Differential Revision: https://reviews.llvm.org/D52664

llvm-svn: 343530