test-includes breaks buildworld when WITHOUT_PF is set in src.conf

test-includes uses pf.h when checking usage of pfvar.h.

But, these lines in include/Makefile remove pf.h when WITHOUT_PF is
set in src.conf:

.if ${MK_PF} != "no"
 INCSGROUPS+=   PF
.endif

This breaks buildworld.  The error message:

In file included from net_pfvar.c:1:
/usr/obj/usr/src/amd64.amd64/tmp/usr/include/net/pfvar.h:65:10: fatal error:
'netpfil/pf/pf.h' file not found
#include <netpfil/pf/pf.h>
         ^~~~~~~~~~~~~~~~~
1 error generated.
--- net_pfvar.o ---
*** [net_pfvar.o] Error code 1

make[3]: stopped in /usr/src/tools/build/test-includes
.ERROR_TARGET='net_pfvar.o'

Removing the .if/.endif fixes it for me, although there may be a better
way to avoid the error.

-- 
Gary Jennejohn