build: fix build failure with -fno-common

build: fix build failure with -fno-common

$ make CCOPTS=-fno-common
gcc ... -o ip
ld: rt_names.o (symbol from plugin): in function "rtnl_rtprot_n2a":
(.text+0x0): multiple definition of "numeric"; ip.o (symbol from plugin):(.text+0x0): first defined here

gcc ... -o tipc
ld: ../lib/libutil.a(utils.o):(.bss+0xc): multiple definition of `pretty';
tipc.o:tipc.c:28: first defined here

References: https://bugzilla.opensuse.org/1160244
Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>