kbuild: fix scripts/adjust_autoksyms.sh* for the no modules case

When CONFIG_TRIM_UNUSED_KSYMS=y and no modules are actually selected,
the adjust_autoksyms.sh script fails with:

sed: can't read .tmp_versions/*.mod: No such file or directory

Let's cope with that case gracefully.
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Michal Marek <mmarek@suse.com>

scripts/adjust_autoksyms.sh

@@ -59,6 +59,7 @@ cat > "$new_ksyms_file" << EOT
  */
 
 EOT
+[ "$(ls -A "$MODVERDIR")" ] &&
 sed -ns -e '3{s/ /\n/g;/^$/!p;}' "$MODVERDIR"/*.mod | sort -u |
 while read sym; do
 	if [ -n "$CONFIG_HAVE_UNDERSCORE_SYMBOL_PREFIX" ]; then