travis: manually generate sys/syscall.h

travis: manually generate sys/syscall.h

Until now, the needed bits were wrongly included in linux/memfd.h

Since Travis' sys/syscall.h doesn't provide the SYS_memfd_create, we
generate that header manually, including the needed bits to avoid
compilation problems, as the ones observed after:
3228335b55c ("intel: aubinator: handle GGTT mappings")

v2: replace fixes commit with the first direct user of
    syscall.h (Emil).

Fixes: 3228335b55c ("intel: aubinator: handle GGTT mappings")
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Juan A. Suarez Romero <jasuarez@igalia.com>
Cc: Dylan Baker <dylan.c.baker@intel.com>
Cc: Eric Engestrom <eric.engestrom@intel.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>