build: disable libxcb extensions
Summary:
Because we pass -qt-xcb to Qt, it will compile in a set of xcb helper libraries and extensions. So skip building all of the libxcb extensions when we build libcxb in depends. More info is available here: https://doc.qt.io/qt-5.9/linux-requirements.html
Backport of core PR16533.
Test Plan: Run the Linux Gitian builds.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D5633