bootstrap.conf: add check for flex

We need the flex to generate new xen_xl_disk parser.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

bootstrap.conf

 # Bootstrap configuration.
 
-# Copyright (C) 2010-2014 Red Hat, Inc.
+# Copyright (C) 2010-2015 Red Hat, Inc.
 
 # This library is free software; you can redistribute it and/or
 # modify it under the terms of the GNU Lesser General Public
@@ -204,6 +204,7 @@ buildreq="\
 autoconf   2.59
 automake   1.9.6
 autopoint  -
+flex       -
 gettext    0.17
 git        1.5.5
 gzip       -