test/rpc: include unistd.h

gcc -c test.c -I ./
test.c: In function ‘recv_resp’:
test.c:18:8: warning: implicit declaration of function ‘read’ [-Wimplicit-function-declaration]
  len = read(socket_fd, buf, MAX_MSG_SIZE);
        ^
test.c: In function ‘send_req’:
test.c:45:6: warning: implicit declaration of function ‘write’ [-Wimplicit-function-declaration]
  if (write(socket_fd, buf, len)  == -1) {
      ^
test.c: In function ‘main’:
test.c:165:2: warning: implicit declaration of function ‘close’ [-Wimplicit-function-declaration]
  close(fd);

Reported-by: Mr Jenkins
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>