zziplib: broaden platforms

Builds fine for FreeBSD and NetBSD.

The use of Python's platforms comes from f2b30a1bf7 ("Zziplib: Build
only where Python is supported."), from way before Nixpkgs supported
cross compilation.  (Python is a native build input.)  Now that we do
support cross compilation, we shouldn't couple the platforms of a
package to one of its native build inputs like this.

pkgs/development/libraries/zziplib/default.nix

@@ -76,6 +76,6 @@ stdenv.mkDerivation rec {
     '';
     license = with licenses; [ lgpl2Plus mpl11 ];
     maintainers = with maintainers; [ AndersonTorres ];
-    platforms = python3.meta.platforms;
+    platforms = platforms.unix;
   };
 }