Summary:
The external Travis CI builds for fbzmq are still using the older
fbcode_builder spec (as opposed to the newer getdeps manifest file).

D16577367 moved the CMakeLists.txt file to the top-level directory, but the
fbzmq spec file was still looking for it in the fbzmq subdirectory.

Reviewed By: jstrizich

Differential Revision: D17005361

fbshipit-source-id: 3f7664eadfb60ec7606124a14445b44ae586b8a7