lxc: Only add /dev/ashmem to config_nodes when it exists

This hasn't existed in mainline Linux kernels since v5.18 and trying to
always mount it regardless just adds to the noise in "waydroid log" in
most cases, so simply don't add it to config_nodes unless it actually
exists.

Silences the following:

  lxc-start: waydroid: ../src/lxc/utils.c: safe_mount: 1221 No such file or directory - Failed to mount "/dev/ashmem" onto "/usr/lib/lxc/rootfs/dev/ashmem"