buffer: add next pointer

Add next pointer to easily make linked lists

pinos/client/buffer.h

@@ -45,6 +45,7 @@ typedef enum {
 } PinosBufferFlags;
 
 struct _PinosBuffer {
+  PinosBuffer *next;
   /*< private >*/
   gsize x[16];
 };

pinos/client/private.h

@@ -62,6 +62,7 @@ typedef struct {
 } PinosStackHeader;
 
 typedef struct {
+  PinosBuffer *next;
   gpointer data;
   gsize size;
   gsize max_size;