callgraph: use bump allocator for callgraph nodes

src/libsysprof/sysprof-callgraph-private.h

@@ -22,6 +22,7 @@
 
 #include <gio/gio.h>
 
+#include "sysprof-allocator-private.h"
 #include "sysprof-callgraph.h"
 #include "sysprof-document.h"
 
@@ -57,6 +58,8 @@ struct _SysprofCallgraph
 {
   GObject                  parent_instance;
 
+  SysprofAllocator        *allocator;
+
   SysprofDocument         *document;
   GListModel              *traceables;