TL;DR: this allows to check if printf argument types are valid.

Apparently, gcc is not able to check if the printf arguments
are in sync with the format string, it a string is not a literal.
This can be seen by compiling the code with -Wformat-nonliteral:

  CC       criu/netfilter.o
criu/netfilter.c: In function ‘nf_connection_switch_raw’:
criu/netfilter.c:80:4: error: format not a string literal, argument
types not checked [-Werror=format-nonliteral]
    dip, (int)dst_port, sip, (int)src_port);

Unfortunately we can't just add -Wformat-nonliteral to CFLAGS as there
is at least one other place in the code what uses non-literal string
as a format string for printf-like function. In this very case, though,
there is no need to use a non-literal, so change it to a define.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>