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
diff --git a/common/rfb/UserPasswdGetter.h b/common/rfb/UserPasswdGetter.h
index 18b0bae..aa72c1d 100644
--- a/common/rfb/UserPasswdGetter.h
+++ b/common/rfb/UserPasswdGetter.h
@@ -25,6 +25,8 @@
     // case no user name will be retrieved.  The caller MUST delete [] the
     // result(s).
     virtual void getUserPasswd(char** user, char** password)=0;
+
+    virtual ~UserPasswdGetter() {}
   };
 }
 #endif