Fix mingw compile error "src/kdeui/kapplication_win.cpp:212:22: error: 'kill' was not declared in this scope"

Summary:
This fixes also similar error with msvc in case openssl development
header has been installed into the same location as KDEWin.

Test Plan: cross compiled with mingw

Reviewers: dfaure, ltoscano, bcooksley

Reviewed By: dfaure

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D9404