QXcbWindow: fix build: getpid() is <unistd.h>

Change-Id: I3494ac2ce660faa49b72fffdc3b9fbb13df0372d
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
(cherry picked from commit ecfa7406f1de34bacca57be2068423206f9c3707)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -41,6 +41,7 @@
 #include <qpa/qwindowsysteminterface.h>
 
 #include <stdio.h>
+#include <unistd.h>
 
 #if QT_CONFIG(xcb_xlib)
 #define register        /* C++17 deprecated register */