Update xdebug to 2.6.1

Summary: Updating xdebug to 2.6.1. All fixed bugs can be found here.

Test Plan: Using visual studio code, run a debugging session with a simple php file.

Reviewers: Triage Team, DataDrake

Reviewed By: Triage Team, DataDrake

Subscribers: DataDrake

Differential Revision: https://dev.solus-project.com/D3516