dovecot: explicitely disable lzma and lz4 support

Avoid linking with liblzma and liblz4 for now.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/packages@39881 3c298f89-4303-0410-b956-a3cf2f4a3e73