It isn't required, __NR_memfd_create is defined in syscall-codes.h.

With this patch, we can run all tests in travis-ci, because there
a kernel has support of memfd, but __NR_memfd_create isn't defined
in installed headers
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>