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

pkgs/os-specific/linux/lockdep/default.nix

@@ -23,6 +23,12 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ flex bison ];
 
+  # 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
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   buildPhase = ''
     make defconfig
     make headers_install