lib/libnetgraph can't compile

[Rumko <rumcic@xxxxxxxxx>]

Due to recent commits, lib/libnetgraph does not compile:
===> lib/libnetgraph
cc -O -pipe -mtune=pentiumpro -Wall  -c /usr/src/lib/libnetgraph/sock.c -o sock.o
cc -O -pipe -mtune=pentiumpro -Wall  -c /usr/src/lib/libnetgraph/msg.c -o msg.o
cc -O -pipe -mtune=pentiumpro -Wall  -c /usr/src/lib/libnetgraph/debug.c -o debug.o
In file included from /usr/obj/usr/src/world_i386/usr/include/net/bpf.h:55,
                 from /usr/src/lib/libnetgraph/debug.c:52:
/usr/obj/usr/src/world_i386/usr/include/sys/mbuf.h:86: error: field `mh_netmsg' has incomplete type
*** Error code 1

Stop in /usr/src/lib/libnetgraph.
*** Error code 1