graphics/qgis: Use gettid patch from qgis project

A few hours after the commit of v3.42.0 of the QGIS port, a suitable
patch [1] was published in the main branch of the QGIS project, which
represents a more general solution for the replacement of the
linux-centric gettid().

[1] https://github.com/qgis/QGIS/commit/00c4fa69116bda5a40af1dfc2ed194f9aec08d52