reducer_vector.h is not included in libcilkrts5

[jmftrindade]

Hi there! Unsure if this is the right place to file this, but reducer_vector.h is missing from libcilkrts5 on Ubuntu Xenial. On the latest xenial version of the lib (5.4.0-6ubuntu1~16.04.11), all but vector reducer headers are available:
https://packages.ubuntu.com/xenial-updates/amd64/libgcc-5-dev/filelist

Perhaps the cilk install instructions at http://cilk.mit.edu/ could be updated to reflect that reducer_vector.h is missing on the latest package for some platforms? And maybe provide libcilkrts5 in the tapir toolchain ppa listed at cilk.mit.edu.

Thanks!
Joana