Settingsaccessor: Add missing include

I hope this will fix the build on the Mac buildbot.

Change-Id: I47169f7baee96b64980d42dea9b540b4fe9e1cbc
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/projectexplorer/settingsaccessor.h

@@ -32,6 +32,7 @@
 
 #include <utils/fileutils.h>
 
+#include <QHash>
 #include <QVariantMap>
 #include <QMessageBox>