botan: disable for static builds

it tries to include <dlfcn.h>

see http://autobuild.buildroot.net/results/2d1/2d1563e253b669b45c9df449c5b3a750c04a43dc/



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