selftests: add missing include unistd

Compiling rxtimestamp.c generates error messages due to
non-existing declaration for write() library call.

Add missing unistd.h include to provide the declaration and
silence the error.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>