Unbreak the build on sparc64: it was caused by #include <net/pfvar.h> that
was conflicting with C++ headers, leading to the following errors:

In file included from /usr/include/c++/4.2/iostream:45,
                 from ../include/XmlFileWriter.h:36,
                 from pflogx.cpp:36:
/usr/include/c++/4.2/ostream:566:47: error: macro "flush" passed 2
arguments, but takes just 1

This header was required before PF_* enums were moved to <netpfil/pf/pf.h>,
which applies to all supported versions of FreeBSD. Thus, do not #include
offending and nowadays useless header file, only #include <netpfil/pf/pf.h>
(which is #included by <net/pfvar.h>) when its definitions are needed.

Tested on: i386, amd64, powerpc, sparc64