Fix a bunch of Hydra evaluation errors

pkgs/tools/X11/winswitch/default.nix

@@ -38,6 +38,8 @@ let
     '';
 
     doCheck = false;
+
+    meta.platforms = stdenv.lib.platforms.mesaPlatforms;
   };
 in stdenv.lib.overrideDerivation base (b: {
   postFixup = b.postFixup + ''

pkgs/tools/X11/xpra/default.nix

@@ -40,5 +40,6 @@ buildPythonPackage rec {
   meta = {
     homepage = http://xpra.org/;
     description = "Persistent remote applications for X";
+    platforms = stdenv.lib.platforms.mesaPlatforms;
   };
 }

pkgs/tools/filesystems/nixpart/default.nix

@@ -24,5 +24,6 @@ in buildPythonPackage rec {
     description = "NixOS storage manager/partitioner";
     license = stdenv.lib.licenses.gpl2Plus;
     maintainers = [ stdenv.lib.maintainers.aszlig ];
+    platforms = stdenv.lib.platforms.linux;
   };
 }

pkgs/tools/networking/mu/default.nix

@@ -33,7 +33,7 @@ stdenv.mkDerivation rec {
     description = "A collection of utilties for indexing and searching Maildirs";
     license = "GPLv3+";
     homepage = "http://www.djcbsoftware.nl/code/mu/";
-    platforms = stdenv.lib.platforms.all;
+    platforms = stdenv.lib.platforms.mesaPlatforms;
     maintainers = with stdenv.lib.maintainers; [ antono the-kenny ];
   };
 }