srecord: add libtool dependency

srecord does not build if glibtool is not available at build time.
The libtool package installs glibtool into /usr/local/bin, so srecord
needs to list this dependency to overcome superenv.

Closes Homebrew/homebrew#15688.

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

Formula/srecord.rb

@@ -5,6 +5,7 @@ class Srecord < Formula
   url 'http://srecord.sourceforge.net/srecord-1.60.tar.gz'
   sha1 '0e0e94e735578346138c916117a8d6c8324e9fec'
 
+  depends_on :libtool => :build
   depends_on 'boost'
   depends_on 'libgcrypt'