Botan uses dlopen & family, so it must link to -ldl

Indirect lib dependencies are not allowed. Code using Botan needs to
link directly to libdl.

Change-Id: I96bde3adf2aef9fdc60af1a70f70e17a79b24048
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>