Switch off httplib in minimal configuration

Summary:
MITK_USE_httplib is on by default which will in turn require OpenSSL, which should not be a requirement for minimal builds or build configurations deriving from Miminal.cmake like CoreCmdApps.cmake.

Fixes T30491.

Test Plan: Already tested with CoreCmdApps build on CentOS 7.

Reviewers: floca, O1 MITK Reviewer Group I

Reviewed By: floca, O1 MITK Reviewer Group I

Maniphest Tasks: T30491

Differential Revision: https://phabricator.mitk.org/D977