LOG4NET-318 add [SecuritySafeCritical] attribute to the Attach method of the RemoteLoggingServerPlugin

According to http://stackoverflow.com/questions/8297419/log4net-doesnt-pass-verification-when-compiling this should fix
the PEVerify issue reported.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4net/trunk@1486895 13f79535-47bb-0310-9956-ffa450edef68