Add an empty virtual destructor to OutOfBandPlugin

I encountered this during dpkg-buildpackage on Debian Jessie:
outofband.h:118:9: error: ‘class Network::OutOfBandPlugin’ has virtual functions and accessible non-virtual destructor [-Werror=non-virtual-dtor]

Author: jkraemer

src/network/outofband.cc

@@ -321,3 +321,5 @@ string OutOfBandCommunicator::read(size_t len) {
   stream_buf = stream_buf.substr(len);
   return rv;
 }
+
+OutOfBandPlugin::~OutOfBandPlugin() { }

src/network/outofband.h

@@ -125,6 +125,7 @@ namespace Network {
     virtual void close_sessions( void ) = 0;
     virtual void shutdown( void ) = 0;
     virtual void attach_oob(Network::OutOfBand *oob_ctl) = 0;
+    virtual ~OutOfBandPlugin() = 0;
 
     friend class OutOfBand;
   };