Fix cross-compiling with MinGW (MXE)
Summary:
While cross-compiling Kdenlive with MXE (mingw64), I don't get pid_t properly defined.
This patch solves the situation on my side
Test Plan: I only tested the cross-building, maybe someone having Windows MinGW development environment could check it doesn't break
Reviewers: vonreth, habacker
Reviewed By: vonreth
Subscribers: Frameworks
Tags: Frameworks, Kdenlive
Differential Revision: https://phabricator.kde.org/D8428