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, 4 insertions, 0 deletions

diff --git a/common/rfb/SSecurityPlain.h b/common/rfb/SSecurityPlain.h
index 2c08c24e..4bf42b74 100644
--- a/common/rfb/SSecurityPlain.h
+++ b/common/rfb/SSecurityPlain.h
@@ -38,6 +38,8 @@ namespace rfb {
       { return validUser(username) ? validateInternal(sc, username, password) : false; }
     static StringParameter plainUsers;
 
+    virtual ~PasswordValidator() { }
+
   protected:
     virtual bool validateInternal(SConnection* sc, const char *username, const char *password)=0;
     static bool validUser(const char* username);
@@ -50,6 +52,8 @@ namespace rfb {
     virtual int getType() const { return secTypePlain; };
     virtual const char* getUserName() const { return username.buf; }
 
+    virtual ~SSecurityPlain() { }
+
   private:
     PasswordValidator* valid;
     unsigned int ulen, plen, state;