build: Fix link errors on some systems

build: Fix link errors on some systems

Since moving get_rate() and get_size() from tc to lib, on some
systems we fail to link because of missing math lib.
Move the functions that require math lib to their own c file
and add -lm to dcb that now use those functions.

../lib/libutil.a(utils.o): In function `get_rate':
utils.c:(.text+0x10dc): undefined reference to `floor'
../lib/libutil.a(utils.o): In function `get_size':
utils.c:(.text+0x1394): undefined reference to `floor'
../lib/libutil.a(json_print.o): In function `sprint_size':
json_print.c:(.text+0x14c0): undefined reference to `rint'
json_print.c:(.text+0x14f4): undefined reference to `rint'
json_print.c:(.text+0x157c): undefined reference to `rint'

Fixes: f3be0e6366ac ("lib: Move get_rate(), get_rate64() from tc here")
Fixes: 44396bdfcc0a ("lib: Move get_size() from tc here")
Fixes: adbe5de96662 ("lib: Move sprint_size() from tc here, add print_size()")
Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Tested-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>