Get AIX 6.1 compiling again by making msg_t class explicit

Older versions of gcc have problems with in-line forward declarations
when there's a naming conflict with a global symbol.

Signed-off-by: AJ Lewis <aj.lewis@quantum.com>

Expand the original patch to all such forward declarations.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/decoder.hpp

@@ -34,6 +34,8 @@
 namespace zmq
 {
 
+    class session_base_t;
+
     //  Helper base class for decoders that know the amount of data to read
     //  in advance at any moment. Knowing the amount in advance is a property
     //  of the protocol used. 0MQ framing protocol is based size-prefixed
@@ -193,7 +195,7 @@ namespace zmq
         decoder_t (size_t bufsize_, int64_t maxmsgsize_);
         ~decoder_t ();
 
-        void set_session (class session_base_t *session_);
+        void set_session (zmq::session_base_t *session_);
 
     private:
 
@@ -202,7 +204,7 @@ namespace zmq
         bool flags_ready ();
         bool message_ready ();
 
-        class session_base_t *session;
+        zmq::session_base_t *session;
         unsigned char tmpbuf [8];
         msg_t in_progress;