net/virtio-user: fix build on Suse 11

net/virtio-user: fix build on Suse 11

On some older systems, such as SUSE 11, the compiling error shows
as:
   .../dpdk/drivers/net/virtio/virtio_user/virtio_user_dev.c:67:22:
         error: ‘O_CLOEXEC’ undeclared (first use in this function)

The fix is to use EFD_CLOEXEC, which is defined in sys/eventfd.h,
instead of O_CLOEXEC which needs _GNU_SOURCE defined on some old
systems.

Fixes: 37a7eb2ae816 ("net/virtio-user: add device emulation layer")

Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>