Fix build for macos

[livelazily]

PF header file urls in Makefile are outdated, faild to build on macOS mojave.

Update this urls to make build success.

[luckypoem]

@livelazily

git clone https://github.com/HaoH/redsocks redsocks-by-HaoH
cd redsocks-by-HaoH
make OSX_VERSION=master DISABLE_SHADOWSOCKS=true
显示：
…
base.c:41:11: fatal error: ‘net/pfvar.h’ file not found
include <net/pfvar.h>
^
1 error generated.
make: *** [base.o] Error 1

我在这里https://opensource.apple.com/source/xnu/xnu-3789.70.16/bsd/net/pfvar.h.auto.html看到pfvar.h文件的内容，但是不知该把它复制到哪里去。

非常感谢你的回复