iftop: add -fcommon workaround

Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:

    ld: tui.o:/build/iftop-1.0pre4/ui_common.h:41: multiple definition of `service_hash';
      iftop.o:/build/iftop-1.0pre4/ui_common.h:41: first defined here

pkgs/tools/networking/iftop/default.nix

@@ -19,6 +19,12 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ncurses libpcap];
 
+  # Workaround build failure on -fno-common toolchains like upstream
+  # gcc-10. Otherwise build fails as:
+  #   ld: tui.o:/build/iftop-1.0pre4/ui_common.h:41: multiple definition of `service_hash';
+  #     iftop.o:/build/iftop-1.0pre4/ui_common.h:41: first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   passthru.tests = { inherit (nixosTests) iftop; };
 
   meta = with lib; {