babeld: fix build on Fedora Rawhide

babeld: fix build on Fedora Rawhide

Fixes the following linker errors:
/usr/bin/ld: babeld/libbabel.a(babel_interface.o):/home/ruben/src/frr/babeld/util.h:125: multiple definition of `v4prefix'; babeld/babel_main.o:/home/ruben/src/frr/babeld/util.h:125: first defined here
/usr/bin/ld: babeld/libbabel.a(babel_zebra.o):/home/ruben/src/frr/babeld/util.h:125: multiple definition of `v4prefix'; babeld/babel_main.o:/home/ruben/src/frr/babeld/util.h:125: first defined here
/usr/bin/ld: babeld/libbabel.a(babeld.o):/home/ruben/src/frr/babeld/util.h:125: multiple definition of `v4prefix'; babeld/babel_main.o:/home/ruben/src/frr/babeld/util.h:125: first defined here
/usr/bin/ld: babeld/libbabel.a(kernel.o):/home/ruben/src/frr/babeld/util.h:125: multiple definition of `v4prefix'; babeld/babel_main.o:/home/ruben/src/frr/babeld/util.h:125: first defined here
/usr/bin/ld: babeld/libbabel.a(message.o):/home/ruben/src/frr/babeld/util.h:125: multiple definition of `v4prefix'; babeld/babel_main.o:/home/ruben/src/frr/babeld/util.h:125: first defined here
/usr/bin/ld: babeld/libbabel.a(neighbour.o):/home/ruben/src/frr/babeld/util.h:125: multiple definition of `v4prefix'; babeld/babel_main.o:/home/ruben/src/frr/babeld/util.h:125: first defined here
/usr/bin/ld: babeld/libbabel.a(net.o):/home/ruben/src/frr/babeld/util.h:125: multiple definition of `v4prefix'; babeld/babel_main.o:/home/ruben/src/frr/babeld/util.h:125: first defined here
/usr/bin/ld: babeld/libbabel.a(resend.o):/home/ruben/src/frr/babeld/util.h:125: multiple definition of `v4prefix'; babeld/babel_main.o:/home/ruben/src/frr/babeld/util.h:125: first defined here
/usr/bin/ld: babeld/libbabel.a(route.o):/home/ruben/src/frr/babeld/util.h:125: multiple definition of `v4prefix'; babeld/babel_main.o:/home/ruben/src/frr/babeld/util.h:125: first defined here
/usr/bin/ld: babeld/libbabel.a(source.o):/home/ruben/src/frr/babeld/util.h:125: multiple definition of `v4prefix'; babeld/babel_main.o:/home/ruben/src/frr/babeld/util.h:125: first defined here
/usr/bin/ld: babeld/libbabel.a(util.o):/home/ruben/src/frr/babeld/util.h:125: multiple definition of `v4prefix'; babeld/babel_main.o:/home/ruben/src/frr/babeld/util.h:125: first defined here
/usr/bin/ld: babeld/libbabel.a(xroute.o):/home/ruben/src/frr/babeld/util.h:125: multiple definition of `v4prefix'; babeld/babel_main.o:/home/ruben/src/frr/babeld/util.h:125: first defined here
/usr/bin/ld: babeld/libbabel.a(babel_filter.o):/home/ruben/src/frr/babeld/util.h:125: multiple definition of `v4prefix'; babeld/babel_main.o:/home/ruben/src/frr/babeld/util.h:125: first defined here
collect2: error: ld returned 1 exit status
make[1]: *** [Makefile:6015: babeld/babeld] Error 1

Signed-off-by: Ruben Kerkhof <ruben@rubenkerkhof.com>