stoken: Fix build for Linux

- This formula is a dependency of `openconnect`, which failed when I
  tried to `brew install openconnect`.
- The error was:
  ```
  src/sdtid.c:32:27: fatal error: libxml/parser.h: No such file or directory
  ```

Closes #13605.

Signed-off-by: Michka Popoff <michkapopoff@gmail>

Formula/stoken.rb

@@ -14,6 +14,7 @@ class Stoken < Formula
 
   depends_on "pkg-config" => :build
   depends_on "nettle"
+  depends_on "libxml2" unless OS.mac?
 
   def install
     args = %W[