QTest::CrashHandler: add missing #includes

Amends c0014becca.

 <string.h> for sigabbrev_np()
 <qnamespace.h> for Qt::Initialization

Change-Id: I95097256825904997ad0fffd0e5f6c5907f8b383
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
(cherry picked from commit 22764dd71515e75748455b2aaf1a26052fddf9a3)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/testlib/qtestcrashhandler_p.h

@@ -15,6 +15,7 @@
 #ifndef QTESTCRASHHANDLER_H
 #define QTESTCRASHHANDLER_H
 
+#include <QtCore/qnamespace.h>
 #include <QtTest/qttestglobal.h>
 
 #include <QtCore/private/qtools_p.h>
@@ -23,6 +24,7 @@
 #include <signal.h>
 #include <sys/mman.h>
 #include <sys/uio.h>
+#include <string.h>
 #include <unistd.h>
 #endif