install: Do post install task only on windows
install: Do post install task only on windows

On my GNU/Linux system, I observed those log on npm install:

    NODE_ENV=production npm install
    > iotivity-node@1.3.0-3 postinstall /tmp/iotivity-node
    > node build-scripts/postinstall.js
    mv: no such file or directory: iotivity-node/build/Release/octbstack.dll
    mv: no such file or directory: iotivity-node/octbstack.dll

It's not blocking, but can be missleading,
when troubleshooting issues while trying iot-rest-api-server.

Signed-off-by: Philippe Coval <philippe.coval@osg.samsung.com>

Closes gh-158

gabrielschulhof/iotivity-node,gabrielschulhof/iotivity-node,otcshare/iotivity-node,gabrielschulhof/iotivity-node,otcshare/iotivity-node,otcshare/iotivity-node