package/dovecot: add optional support for lz4

When lz4 was compiled before, dovecot will use it as optional dependency:

$ output/host/usr/bin/i586-buildroot-linux-uclibc-readelf -a output/target/usr/lib/dovecot/lib30_imap_zlib_plugin.so | grep NEEDED
 0x00000001 (NEEDED)                     Shared library: [libz.so.1]
 0x00000001 (NEEDED)                     Shared library: [liblzma.so.5]
 0x00000001 (NEEDED)                     Shared library: [liblz4.so.1]
 0x00000001 (NEEDED)                     Shared library: [libc.so.1]

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