treewide: Add lots of meta.platforms

Build-tested on x86_64 Linux & Mac.

pkgs/development/libraries/readline/5.x.nix

@@ -13,5 +13,6 @@ stdenv.mkDerivation {
   patches = stdenv.lib.optional stdenv.isDarwin ./shobj-darwin.patch;
   meta = {
     branch = "5";
+    platforms = stdenv.lib.platforms.unix;
   };
 }

pkgs/development/libraries/readline/6.2.nix

@@ -50,6 +50,7 @@ stdenv.mkDerivation (rec {
 
     maintainers = [ ];
     branch = "6.2";
+    platforms = stdenv.lib.platforms.unix;
   };
 }