xidel: Set platforms to platforms.all

The Linux-only `platforms` setting dates to 10 years ago in 7f06428544, where it was accompanied by the comment

> `# more platforms will be supported when we switch to source build`

That switch happened in 2021 in d14ea1499b. I've confirmed that the current package builds just fine on my `aarch64-darwin` laptop.

pkgs/tools/text/xidel/default.nix

@@ -90,7 +90,7 @@ in stdenv.mkDerivation rec {
     mainProgram = "xidel";
     homepage = "https://www.videlibri.de/xidel.html";
     license = licenses.gpl3Plus;
-    platforms = platforms.linux;
+    platforms = platforms.all;
     maintainers = [ maintainers.bjornfor ];
   };
 }