nut: now needs C++ support

Fixes http://autobuild.buildroot.net/results/8c5/8c5dc5f09e2dcb2a3aba1c451a53de8e79d531dc/



After the version bump, nut now unconditionally builds a C++ client library,
so adjust the dependencies to match.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>