Log Message:
sysutils/pfstat: Add DragonFly hack

After spending more time on this this trying to patch it properly than I
care to admit, I concede defeat and add a hack.mk for DragonFly.

I'm not exactly sure what the problem is, other than "make" works on the
subdir makefile and "bmake" doesn't.  And that's only true if executed
outside of the package -- hardcoding "make" inside the do-build phase
doesn't work.  Weird problem, but the hack works.