debuggee_qlistcontainer: build and test deprecated QLinkedList

3086f6f5 raised Qt deprecation level to
5.15 and therefore disabled building QLinkedList deprecated in Qt 5.15.
This causes the following test_gdbprinters and test_lldbformatters
failures:
QWARN  : QtPrintersTest::testQListContainer(QLinkedList) FOO "No symbol \"intList\" in current context."
FAIL!  : QtPrintersTest::testQListContainer(QLinkedList) 'out.contains(QString("%1<int> (size = 0)").arg(container).toLocal8Bit())' returned FALSE. ()
   Loc: [kdevelop/plugins/gdb/unittests/test_gdbprinters.cpp(163)]
FAIL!  : LldbFormattersTest::testQListContainer(QLinkedList) Session ended before reaching state 3
   Loc: [kdevelop/plugins/lldb/unittests/test_lldbformatters.cpp(377)]