qqmldebugclient.cpp: Fix warning about missing return.

In file included from ../shared/qqmldebugclient.cpp:473:0:
../shared/qqmldebugclient.cpp: In member function
'QString QQmlDebugClient::stateString() const':
../shared/qqmldebugclient.cpp:452:1: warning: control reaches end of
non-void function

Change-Id: Ic5422dcff41591691621077ade45b6ea3ea3a8f1
Reviewed-by: Kai Koehne <kai.koehne@digia.com>