netgraph(3) points to #include <netgraph/netgraph.h>, which is kernel only.
The man page refers to the user-space part of the netgraph module, which is located in <netgraph.h>.

#include <netgraph/netgraph.h>

compile as use space program will produce the error message

/usr/include/netgraph/netgraph.h:48:2: error: "This file should not be included in user level programs"

Change it to

#include <netgraph.h>

and it compiles fine.