sshfs: Added openssh-client dependency

Notes:
The default ssh client on OpenWrt is provided by dropbear.
Dropbear initially works with sshfs (after some command-line hacks), but will sometimes cause random data-loss. I assume this happens when the Tx buffer is full, which happens quite often with low upload speeds.
In an attempt to fix this I tried disabling multi-threading, sync-writes, sync-reads, debug options, etc... Nothing helped or had any major influence on the problem.

Only after changing ssh from dropbear to openssh-client, sshfs works without any problems on OpenWrt. No data loss, reliable and fast.