gnutls: requires WCHAR support

gnutls uses wctomb(), which is available only on C libraries with
wide-char support. This fixes the following build failure:

 http://autobuild.buildroot.org/results/cd4e73be80fbb64858f4cf911d2b893b0fc06465/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>