Merge #12783: add some meta.platforms attributes

pkgs/development/compilers/dtc/default.nix

···
     homepage = https://git.kernel.org/cgit/utils/dtc/dtc.git;
     license = licenses.gpl2; # dtc itself is GPLv2, libfdt is dual GPL/BSD
     maintainers = [ maintainers.dezgeg ];
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }

pkgs/development/libraries/acl/default.nix

···
   meta = {
     homepage = http://savannah.nongnu.org/projects/acl;
     description = "Library and tools for manipulating access control lists";
+    platforms = stdenv.lib.platforms.linux;
   };
 }

pkgs/development/libraries/attr/default.nix

···
   meta = {
     homepage = http://savannah.nongnu.org/projects/attr/;
     description = "Library and tools for manipulating extended attributes";
+    platforms = stdenv.lib.platforms.linux;
   };
 }

pkgs/tools/X11/ckbcomp/default.nix

···
     homepage = http://anonscm.debian.org/cgit/d-i/console-setup.git;
     license = licenses.gpl2Plus;
     maintainers = with stdenv.lib.maintainers; [ dezgeg ];
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }