channels: ssh-agnet: cmake: turn off server side channel by default in `ChannelsOptions.cmake`
CMake v3.5.1 throws the following error if this is not set:
```
CMake Error at channels/CMakeLists.txt:48 (if):
if given arguments:
"OFF" "OR"
Unknown arguments specified
Call Stack (most recent call first):
channels/sshagent/ChannelOptions.cmake:6 (define_channel_options)
channels/CMakeLists.txt:273 (include)
```
set(OPTION_DEFAULT OFF)
set(OPTION_CLIENT_DEFAULT OFF)
+set(OPTION_SERVER_DEFAULT OFF)
define_channel_options(NAME "sshagent" TYPE "dynamic"
DESCRIPTION "SSH Agent Forwarding (experimental)"
