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

pkgs/tools/networking/oonf-olsrd2/default.nix

@@ -15,6 +15,13 @@ stdenv.mkDerivation rec {
     hash = "sha256-7EH2K7gaBGD95WFlG6RRhKEWJm91Xv2GOHYQjZWuzl0=";
   };
 
+  # 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
+  # Can be removed once release with https://github.com/OLSR/OONF/pull/40 is out.
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   cmakeFlags = [
     "-DOONF_NO_WERROR=yes"
   ];