curl 7.15: Disable on Darwin

Build is failing and this version of curl is not used for anything
useful on Darwin, so it's not worth fixing.

pkgs/tools/networking/curl/7.15.nix

···
   meta = {
     homepage = "http://curl.haxx.se/";
     description = "A command line tool for transferring files with URL syntax";
-    platforms = stdenv.lib.platforms.all;
+    platforms = with stdenv.lib.platforms; allBut darwin;
   };
 }