package/icu: icu-uc: provide -lstdc++ flag for static build

icu-uc.pc must provide -lstdc++ for C application linked
statically with libicuuc.a.

Related to:
http://autobuild.buildroot.net/results/210/2107f9dfb39eeb6559fb4271c7af8b39aef521ca/



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>