Error at the time of compiling netmap-fwd on -CURRENT

[John Jasen]

FreeBSD 11 and prior, dprintf in stdio.h had to be specifically called
by setting particular defines.

These were removed in -CURRENT, which caused dprintf from
/usr/include/stdio.h to coflict with the declarations in netmap-fwd.
Commenting out the #define and the int dprintf statements in
netmap-fwd/util.h and util.c allowed it to compile and mostly work.