Do not leak XVisualInfo (GLX path)

Summary: XVisualInfo is not released by get_gl_info_glx()

Test Plan: Leak check with valgrind shows that visinfo is no longer leaking.

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6567