package/dovecot: Add optional support for icu

Optional dependency added to fts plugin since Dovecot 2.2.17:
http://hg.dovecot.org/dovecot-2.2/diff/b179bbd226e5/configure.ac



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>