docs/xen-headers: allow headers to be symlinks

There's no apparent reason not to permit this, and since we don't
support out-of-source-tree builds, the least overhead way of doing
multiple, differently configured (perhaps different architecture)
builds from a single source tree is to create symlinked build trees.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/docs/xen-headers
+++ b/docs/xen-headers
@@ -368,7 +368,7 @@ foreach $pass (qw(1 2)) {
     find({ wanted => 
                sub {
                    return unless m/\.h$/;
-                   lstat $File::Find::name or die "$File::Find::name $!";
+                   stat $File::Find::name or die "$File::Find::name $!";
                    -f _ or die "$File::Find::name";
                    substr($File::Find::name, 0, 1+length $basedir) 
                        eq "$basedir/"