rfb: Fix struct-vs-class warnings

Clang doesn't like when struct and class are used to refer
to the same thing interchangeably.

Fixes all instances of:

In file included from /home/shade/dev/tigervnc/common/rfb/CConnection.cxx:29:
In file included from /home/shade/dev/tigervnc/common/rfb/CConnection.h:28:
/home/shade/dev/tigervnc/common/rfb/DecodeManager.h:35:3: error: class 'Exception' was previously declared as a struct [-Werror,-Wmismatched-tags]
  class Exception;
  ^
/home/shade/dev/tigervnc/common/rdr/Exception.h:32:10: note: previous use is here
  struct Exception {
         ^
/home/shade/dev/tigervnc/common/rfb/DecodeManager.h:35:3: note: did you mean struct here?
  class Exception;
  ^~~~~
  struct
/home/shade/dev/tigervnc/common/rfb/DecodeManager.h:43:3: error: class 'Rect' was previously declared as a struct [-Werror,-Wmismatched-tags]
  class Rect;
  ^
/home/shade/dev/tigervnc/common/rfb/Rect.h:68:10: note: previous use is here
  struct Rect {
         ^
/home/shade/dev/tigervnc/common/rfb/DecodeManager.h:43:3: note: did you mean struct here?
  class Rect;
  ^~~~~
  struct
diff --git a/common/rfb/DecodeManager.h b/common/rfb/DecodeManager.h
index fbb7f77..2098063 100644
--- a/common/rfb/DecodeManager.h
+++ b/common/rfb/DecodeManager.h
@@ -32,7 +32,7 @@
 }
 
 namespace rdr {
-  class Exception;
+  struct Exception;
   class MemOutStream;
 }
 
@@ -40,7 +40,7 @@
   class CConnection;
   class Decoder;
   class ModifiablePixelBuffer;
-  class Rect;
+  struct Rect;
 
   class DecodeManager {
   public: