botan: requires a toolchain with threads

fixes http://autobuild.buildroot.org/results/3df/3dfc002d8c19001dea4f13aa06cc6ec3f65fc4cb/

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>