_0verkill: add -fcommon workaround for gcc-10

Without the change the build against upstream gcc-10 build fails as:

ld: sprite.o:/build/0verkill/data.h:171: multiple definition of `obj_attr'; server.o:/build/0verkill/data.h:171: first defined here
ld: sprite.o:/build/0verkill/data.h:142: multiple definition of `weapon_name'; server.o:/build/0verkill/data.h:142: first defined here

Use -fcommon as a workeround until upstream updates the code.

pkgs/games/0verkill/default.nix

···
     autoupdate
   '';
 
+  # The code needs an update for gcc-10:
+  #   https://github.com/hackndev/0verkill/issues/7
+  NIX_CFLAGS_COMPILE = "-fcommon";
   hardeningDisable = [ "all" ]; # Someday the upstream will update the code...
 
   meta = with lib; {