Move the use_iec declaration to the tools

Move the use_iec declaration to the tools

The tools "ip" and "tc" use a flag "use_iec", which indicates whether, when
formatting rate values, the prefixes "K", "M", etc. should refer to powers
of 1024, or powers of 1000. The flag is currently kept as a global variable
in "ip" and "tc", but is nonetheless declared in util.h.

Instead, move the declaration to tool-specific headers ip/ip_common.h and
tc/tc_common.h.

Signed-off-by: Petr Machata <me@pmachata.org>
Signed-off-by: David Ahern <dsahern@gmail.com>