Do not leak XVisualInfo (X11 EGL path)

Summary: XVisualInfo is not released by get_gl_info_egl

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

Reviewers: graesslin

Reviewed By: graesslin

Subscribers: graesslin, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6566