Bug 31356

pfil_head_t seems to be a typedef'd type according to its name, 
but is actually declared as a variable in sys/net/pfil.h.

"typedef" seems dropped before "struct":
(from sys/net/pfil.h)
struct pfil_head {  
        pfil_list_t     ph_in;
        pfil_list_t     ph_out;
        int             ph_init;
} pfil_head_t;

Fix: 

Append "typedef" before "struct" at line 57 in sys/net/pfil.h (rev. 1.6).
How-To-Repeat: Compile and link these two files (a.c and b.c) with c++ compiler:
  c++ a.c b.c

This results in:
/tmp/cc5njlvK.o(.data+0x0): multiple definition of `pfil_head_t'
/tmp/ccaxafCz.o(.data+0x0): first defined here

a.c:
#include <sys/types.h>
#include <net/pfil.h>
int main(void) { return 0; }

b.c:
#include <sys/types.h>
#include <net/pfil.h>