Fix building problems in extra::unicode.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #8260 : omasanori/rust/fix-extra-unicode, r=pcwalton

WIth this patch `RUSTFLAGS='--cfg unicode' make check"` passed successfully.

* Why doesn't `#[link_name="icuuc"]` make libextra to link against libicuuc.so?
* In `extra::unicode::tests`, `use unicode; unicode::is_foo('a')` failed but `use unicode::*; is_foo('a')` succeeded. Is it right?