depends: Do not force Precompiled Headers (PCH) for building Qt on Linux

Summary:
On CentOS 8 the forced '-pch' option breaks Qt build.
Removing '-pch' option does not affect build time for other Linux
systems.

This is a backport of core#20520

Test Plan: gitian builds, local crossbuild

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13932