Fix include path to kjob.h

Summary:
Fix include path to kjob.h. When kcoreaddons is installed all the
headers files are installed in the same directory, so we shouldn't use
jobs/ when including files from public header files.

Reviewers: dfaure, meven, vkrause

Reviewed By: vkrause

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24434