scripts/clean-includes: Skip symbolic links

When a symbolic link points to a file that needs cleaning, the script
replaces the link with a cleaned regular file.  Not wanted; skip them.

We have a few symbolic links under subprojects/libvduse/ and
subprojects/libvhost-user/.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20230202133830.2152150-4-armbru@redhat.com>

scripts/clean-includes

@@ -113,6 +113,10 @@ EOT
 
 files=
 for f in "$@"; do
+  if [ -L "$f" ]; then
+      echo "SKIPPING $f (symbolic link)"
+      continue
+  fi
   case "$f" in
     *.c.inc)
       # These aren't standalone C source files