stunnel: fix configure ssl parameter

'--with-ssl=DIR' configure parameter expects the location of the
libraries and include files in the sysroot. This means the directory
containing the 'lib' and 'include' subdirectories, not the include
directory itself.

This fixes an error at configure time:

checking
<local-path>/tmp/sysroots/ccimx6adpt/usr/include/include/openssl/engine.h
usability... no

(notice duplicated include entry in above line)

Signed-off-by: Javier Viguera <javier.viguera@digi.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>