Add linux/userfaultfd.h to criu sources. This header is a part
of the kernel API and I see nothing wrong to have in the repo.

Why we want to do this:
* to check that criu works correctly if a kernel doesn't
  support userfaultfd.
* to check compilation of the userfaultfd part in travis-ci.

v2: remove UFFD from FEATURES_LIST
Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Adrian Reber <areber@redhat.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>