Use absolute imports in GDB scripts

Summary:
Citing pep-0008:
"Implicit relative imports should never be used and have been removed in Python 3."

This fixes problems at least on Arch where gdb is compiled with python3.

Clone of https://github.com/KDE/kde-dev-scripts/pull/2. I was not sure about the reviewer, hope it's correct.

Reviewers: #kde_applications, scarpino

Reviewed By: scarpino

Subscribers: scarpino, ltoscano

Differential Revision: https://phabricator.kde.org/D5823