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 <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20485
Signed-off-by: Álvaro Fernández Rojas <[email protected]>
Paul Donald