package/botan: fix static build with atomic

Fixes:
 - http://autobuild.buildroot.org/results/9db39b209a0f4be1e09b7e4ddb5e9a63da4fbbfa

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
(cherry picked from commit b79439af44591aaf061a4757de43de3374ffde29)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>