oonf-olsrd2: add -fcommon workaround
oonf-olsrd2: add -fcommon workaround

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

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs