Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:
ld: CMakeFiles/oonf_dlep_proxy.dir/router/dlep_router_session.c.o:(.bss+0x0):
multiple definition of `LOG_DLEP_ROUTER'; CMakeFiles/oonf_dlep_proxy.dir/router/dlep_router.c.o:(.bss+0x0): first defined here