Update mcs/class/corlib/System/OperatingSystem.cs

"The ISerializable interface implies a constructor with the signature constructor (SerializationInfo information, StreamingContext context)."
from http://msdn.microsoft.com/en-us/library/system.runtime.serialization.iserializable.aspx
Merge pull request #576 from UCIS/patch-1

Fix ISerializable implementation in mcs/class/corlib/System/OperatingSystem.cs