build: avoid tabs that failed syntax-check

Introduced in commit f299ddd6.

* src/check-symfile.pl: Fix whitespace.
* .dir-locals.el (perl-mode): Prevent future occurrences.

.dir-locals.el

@@ -14,4 +14,7 @@
  (nxml-mode . (
 	       (indent-tabs-mode . nil)
 	       ))
+ (perl-mode . (
+	       (indent-tabs-mode . nil)
+	       ))
  )