HPCC-32840 Fix macOS build break

Reorder CMPException members to match constructor initialization order.

Signed-off-by: Tim Klemm <tim.klemm@lexisnexisrisk.com>

system/mp/mpcomm.cpp

@@ -221,7 +221,6 @@ struct MultiPacketHeader
 
 class DECL_EXCEPTION CMPException: public IMP_Exception, public CInterface
 {
-    StringAttr msg;
 public:
     IMPLEMENT_IINTERFACE;
 
@@ -254,6 +253,7 @@ class DECL_EXCEPTION CMPException: public IMP_Exception, public CInterface
 private:
     MessagePassingError error;
     SocketEndpoint endpoint;
+    StringAttr msg;
 };