libjpeg-turbo: make Hydra only build it on Linux

I don't know what's wrong with darwin.
Anyway, it's unused on darwin by default.

pkgs/development/libraries/libjpeg-turbo/default.nix

@@ -17,8 +17,10 @@ stdenv.mkDerivation rec {
     homepage = http://libjpeg-turbo.virtualgl.org/;
     description = "A faster (using SIMD) libjpeg implementation";
     license = licenses.ijg; # and some parts under other BSD-style licenses
-    platforms = platforms.all;
     maintainers = [ maintainers.vcunat ];
+    # upstream supports darwin (and others), but it doesn't build currently
+    platforms = platforms.all;
+    hydraPlatforms = platforms.linux;
   };
 }