Forward declare 'struct ast_json' in asterisk.h

The ast_json structure is used in many Asterisk headers and is often the
only part of json.h used.  This adds a forward declaration to asterisk.h
and removes the include of json.h from many headers.  The declaration
has been left in endpoints.h and stasis.h to avoid problems with source
files that use ast_json functions without directly including json.h.

ari.h continues to include json.h as it uses enum
ast_json_encoding_format.

Change-Id: Id766aabce6bed56626d27e8d29f559b5e687b769

include/asterisk.h

@@ -317,6 +317,7 @@ struct ast_module;
 struct ast_variable;
 struct ast_str;
 struct ast_sched_context;
+struct ast_json;
 
 /* Some handy macros for turning a preprocessor token into (effectively) a quoted string */
 #define __stringify_1(x)	#x

include/asterisk/channel.h

@@ -178,7 +178,6 @@ extern "C" {
 #include "asterisk/ccss.h"
 #include "asterisk/framehook.h"
 #include "asterisk/stasis.h"
-#include "asterisk/json.h"
 #include "asterisk/endpoints.h"
 
 #define DATASTORE_INHERIT_FOREVER	INT_MAX

include/asterisk/stasis_app.h

@@ -51,7 +51,6 @@
  */
 
 #include "asterisk/channel.h"
-#include "asterisk/json.h"
 
 /*! @{ */
 

include/asterisk/stasis_channels.h

@@ -22,7 +22,6 @@
 
 #include "asterisk/stringfields.h"
 #include "asterisk/stasis.h"
-#include "asterisk/json.h"
 #include "asterisk/channel.h"
 
 /*! \addtogroup StasisTopicsAndMessages

include/asterisk/stasis_endpoints.h

@@ -28,7 +28,6 @@
  */
 
 #include "asterisk/endpoints.h"
-#include "asterisk/json.h"
 #include "asterisk/stasis.h"
 #include "asterisk/stasis_cache_pattern.h"
 #include "asterisk/stringfields.h"

include/asterisk/stasis_system.h

@@ -19,7 +19,6 @@
 #ifndef _ASTERISK_STASIS_SYSTEM_H
 #define _ASTERISK_STASIS_SYSTEM_H
 
-#include "asterisk/json.h"
 #include "asterisk/stasis.h"
 
 /*!