Socket::operator=: Return non-const reference

Found by cppcheck.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/network/network.h

@@ -114,7 +114,7 @@ namespace Network {
       ~Socket();
 
       Socket( const Socket & other );
-      const Socket & operator=( const Socket & other );
+      Socket & operator=( const Socket & other );
     };
 
     std::deque< Socket > socks;