odhcpd: set procd expected variable for odhcpd update script

odhcpd: set procd expected variable for odhcpd update script

Otherwise odhcpd logs:

```
odhcpd[14970]: Sent 113 bytes to xxx%lan@br-lan
BusyBox v1.37.0 (2025-10-10 09:07:48 UTC) multi-call binary.

Usage: basename FILE [SUFFIX] | -a FILE... | -s SUFFIX FILE...

Strip directory path and SUFFIX from FILE

        -a              All arguments are FILEs
        -s SUFFIX       Remove SUFFIX (implies -a)
odhcpd[14970]: Netlink newneigh xxx on lan
```

Apparently procd scripts expect initscript set.

h/t @Alphix for finding this.

Signed-off-by: Paul Donald <newtwen+github@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/20485
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>