When linking test-unicode statically it needs $(ICU_LIBS)
which contains all required flags.
Especially -lstdc++.

Fixes:
http://autobuild.buildroot.net/results/210/2107f9dfb39eeb6559fb4271c7af8b39aef521ca/Signed-off-by: NRomain Naour <romain.naour@openwide.fr>