lockdep: add -fcommon workaround
lockdep: add -fcommon workaround

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

    ld: lockdep.o:/build/linux-5.0.21/tools/lib/lockdep/../../include/linux/rcu.h:5: multiple definition of
      `rcu_scheduler_active'; common.o:/build/linux-5.0.21/tools/lib/lockdep/../../include/linux/rcu.h:5: 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