guile: fix missing :libtool dep

guile has a dep on libtool because it uses `-lltdl`
Add the dep using the new DSL style.
Fixes build errors on ML.

Closes Homebrew/homebrew#13892

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/guile.rb

@@ -13,6 +13,7 @@ class Guile < Formula
   end
 
   depends_on 'pkg-config' => :build
+  depends_on :libtool
   depends_on 'libffi'
   depends_on 'libunistring'
   depends_on 'bdw-gc'