announce-gen: avoid failure due to lack of Digest::SHA1

announce-gen: avoid failure due to lack of Digest::SHA1

Even with the preferred Digest::SHA available, this script
would fail when the backup module, Digest::SHA1, was not installed.
* build-aux/announce-gen: Quote the conditional use of "use".
Reported by Reuben Thomas in:
http://thread.gmane.org/gmane.comp.lib.gnulib.bugs/30444