ostree: Workaround the SIGCHLD handler issue

Glib overwrites forkfd's SIGCHLD handler, this
breaks QProcess.

This workaround solves the issue for ostree use
case. Other parts of the stack are still affected,
as there might some code elsewhere that uses QProcess
and the buggy Glib subprocess handling code.

Change-Id: I7077ad9479d2c7cc6f1f693b8241bd5bcdc640b2
Reviewed-by: Samuli Piippo <samuli.piippo@qt.io>