libAfterImage: pass --without-x explicitly

This fixes build on GitHub Actions macOS machines.

pkgs/development/libraries/libAfterImage/default.nix

···
     "--disable-mmx-optimization"
     "--${if static then "enable" else "disable"}-staticlibs"
     "--${if !static then "enable" else "disable"}-sharedlibs"
-  ] ++ lib.optional withX "--with-x";
+    "--${if withX then "with" else "without"}-x"
+  ];
 
   meta = with lib; {
     homepage = "http://www.afterstep.org/afterimage/";