tslib: set meta.platforms to linux

I noticed it was failing for Darwin on zh.fail. Set platforms to stop it
from building.

Signed-off-by: Ethan Carter Edwards <[e-mail]>

pkgs/by-name/ts/tslib/package.nix

···
     description = "Touchscreen access library";
     homepage = "http://www.tslib.org/";
     license = lib.licenses.lgpl21;
-    platforms = lib.platforms.all;
+    platforms = lib.platforms.linux; # requires linux headers <linux/input.h>
     maintainers = with lib.maintainers; [ shogo ];
     teams = with lib.teams; [ ngi ];
   };