package/libkrb5: add openssl optional dependency

package/libkrb5: add openssl optional dependency

openssl is an optional dependency which is enabled by default since at
least 2007 and
https://github.com/krb5/krb5/commit/4c17f25c0f106ec8e3aad2cee4e7a24f66b5a65d

Enable DES, MD4 and RC4 in openssl to fix build failure raised since
commit a83d41867c8d69a77d5cd0a665aa216af5340359

Fixes:
 - http://autobuild.buildroot.org/results/d73b477bd2064aee076f9debfd8d3346c63ba657

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: squash the two commits together]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>