Fixed dbus-c++ gcc 4.7 building problem.

Just add "unistd.h" inclusion to eventloop-integration.h.

BUG=None
TEST=Manually built using gcc 4.7.

Change-Id: I87bd1f90db6a4c974a5ed8134044e8be2034aff2
Reviewed-on: https://gerrit.chromium.org/gerrit/24260
Tested-by: Han Shen <shenhan@google.com>
Reviewed-by: Yunlian Jiang <yunlian@chromium.org>
Commit-Ready: Han Shen <shenhan@chromium.org>