Merge pull request #28823 from disassembler/crda

crda: fix build with gcc6

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

···
     "REG_BIN=${wireless-regdb}/lib/crda/regulatory.bin"
   ];
 
+  NIX_CFLAGS_COMPILE = "-Wno-error=unused-const-variable";
+
   buildFlags = [ "all_noverify" ];
   enableParallelBuilding = true;