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/CMsgWriter.h b/common/rfb/CMsgWriter.h
index 06ecbe7..f9de72f 100644
--- a/common/rfb/CMsgWriter.h
+++ b/common/rfb/CMsgWriter.h
@@ -33,7 +33,7 @@
 
   class PixelFormat;
   class ConnParams;
-  class ScreenSet;
+  struct ScreenSet;
   struct Rect;
 
   class CMsgWriter : public InputHandler {
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:
diff --git a/common/rfb/Decoder.h b/common/rfb/Decoder.h
index 3840b3f..86ee4ef 100644
--- a/common/rfb/Decoder.h
+++ b/common/rfb/Decoder.h
@@ -27,9 +27,10 @@
 namespace rfb {
   class ConnParams;
   class ModifiablePixelBuffer;
-  class Rect;
   class Region;
 
+  struct Rect;
+
   enum DecoderFlags {
     // A constant for decoders that don't need anything special
     DecoderPlain = 0,
diff --git a/common/rfb/EncodeManager.h b/common/rfb/EncodeManager.h
index 79db950..4319b42 100644
--- a/common/rfb/EncodeManager.h
+++ b/common/rfb/EncodeManager.h
@@ -32,7 +32,7 @@
   class PixelBuffer;
   class RenderedCursor;
   class Region;
-  class Rect;
+  struct Rect;
 
   struct RectInfo;
 
diff --git a/common/rfb/SMsgWriter.h b/common/rfb/SMsgWriter.h
index 548b8e8..6c665b6 100644
--- a/common/rfb/SMsgWriter.h
+++ b/common/rfb/SMsgWriter.h
@@ -32,7 +32,7 @@
 namespace rfb {
 
   class ConnParams;
-  class ScreenSet;
+  struct ScreenSet;
 
   class SMsgWriter {
   public: