unshield: add darwin to platforms

Motivation: Required for Darwin build support for OpenMW. Unshield is a required package.

No changes were required to build on Darwin.

pkgs/tools/archivers/unshield/default.nix

···
     description = "Tool and library to extract CAB files from InstallShield installers";
     homepage = "https://github.com/twogood/unshield";
     license = licenses.mit;
-    platforms = platforms.linux;
+    platforms = platforms.linux ++ platforms.darwin;
   };
 }