Unexport kjscmd

Summary:
This is a user tool, not a build-time tool. Having it exported makes it
required to be available (and runnable) for the corresponding find_package
call to work, which is by default not the case when cross-compiling.

Reviewers: Frameworks, apol

Reviewed By: apol

Subscribers: bshah

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D9589