libs/cyassl: re-enable the stunnel flag
This partially reverts commit 15734b023b7e04d12d258ea28bfb5e6e735f10b7.
--enable-stunnel was actually important and properly described in
commit 9b118cde898e. Removing it broke ustream-cyassl
Signed-off-by: Felix Fietkau <[email protected]>