CMake: distribute hphp/system/ public headers too

Summary: Add auto_sources/HHVM_PUBLIC_HEADERS under hphp/system/ as well, which
was forgotten from the initial build system improvements. This
distributes systemlib.h, among others, which is necessary for building
extensions.
Closes #3085

Reviewed By: @ptarjan

Differential Revision: D1422661

Pulled By: @fredemmott