libmariadb 10.2 needs to be linked in together with iconv. On musl and
glibc iconv is part of libc. That's not the case for uclibc, where
libiconv-full needs to be used. This commit adds -liconv to the linker
flags and aids libzdb in finding libiconv.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>