netshaper: fix build failure

netshaper: fix build failure

netshaper fails to build from sources with this error:

$ make
netshaper
    CC       netshaper.o
    LINK     netshaper
/usr/bin/ld: ../lib/libutil.a(utils_math.o): in function `get_rate':
utils_math.c:(.text+0x97): undefined reference to `floor'
/usr/bin/ld: ../lib/libutil.a(utils_math.o): in function `get_size64':
utils_math.c:(.text+0x2a8): undefined reference to `floor'
collect2: error: ld returned 1 exit status
make[1]: *** [Makefile:10: netshaper] Error 1
make: *** [Makefile:81: all] Error 2

Fix this simply linking against the math C library, similarly to what we
already did with commit 1a22ad2721fb ("build: Fix link errors on some
systems").

Fixes: 6f7779ad4ef6 ("netshaper: Add netshaper command")
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>