Add missing virtual destructors

Fix warnings emitted by Clang:

/home/shade/dev/tigervnc/common/rdr/FdInStream.h:30:9: error: 'rdr::FdInStreamBlockCallback' has virtual functions but non-virtual destructor [-Werror,-Wnon-virtual-dtor]
  class FdInStreamBlockCallback {
        ^
In file included from /home/shade/dev/tigervnc/common/network/TcpSocket.cxx:44:
In file included from /home/shade/dev/tigervnc/common/network/TcpSocket.h:31:
/home/shade/dev/tigervnc/common/network/Socket.h:82:9: error: 'network::ConnectionFilter' has virtual functions but non-virtual destructor [-Werror,-Wnon-virtual-dtor]
  class ConnectionFilter {
        ^
..etc

1 files changed, 1 insertions, 0 deletions

diff --git a/common/rdr/FdInStream.h b/common/rdr/FdInStream.h
index 5d9598c8..b4c87653 100644
--- a/common/rdr/FdInStream.h
+++ b/common/rdr/FdInStream.h
@@ -30,6 +30,7 @@ namespace rdr {
   class FdInStreamBlockCallback {
   public:
     virtual void blockCallback() = 0;
+    virtual ~FdInStreamBlockCallback() {}
   };
 
   class FdInStream : public InStream {