[Qt][Win][WK2] Build fix after r140957.

Rubber-stamped by Csaba Osztrogonác.

<sys/wait.h> is only needed and available on linux therefore it's moved into an ifdef block.

* WebProcess/qt/WebProcessMainQt.cpp:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140964 268f45cc-cd09-0410-ab3c-d52691b4dbfc