service: service should compile on Ubuntu 14.04

I'm not quite sure what the difference is (I have gcc 4.8, but there are
probably also header differences), but when I compile the service on 14.04 I
get:

  CC       cr-service.o
cr-service.c: In function ‘start_page_server_req’:
cr-service.c:536:8: error: ignoring return value of ‘write’, declared with attribute warn_unused_result [-Werror=unused-result]
   write(start_pipe[1], &ret, sizeof(ret));
        ^
cr-service.c:544:6: error: ignoring return value of ‘read’, declared with attribute warn_unused_result [-Werror=unused-result]
  read(start_pipe[0], &ret, sizeof(ret));
      ^

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Tested-by: https://travis-ci.org/avagin/criu/builds/34990769


Signed-off-by: Pavel Emelyanov <xemul@parallels.com>