Fix linking of qmlimportscanner in MSVC debug builds.

Qt5QmlDevToolsd.lib(qqmlirbuilder.obj) : error LNK2019: unresolved external symbol "__declspec(dllimport) public: __cdecl QmlIR::CompiledFunctionOrExpression::CompiledFunctionOrExpression(void)"

Use Q_QML_PRIVATE_EXPORT so that it does not have effect when building
qmldevtools.

Change-Id: I138fe9cba518829d7ec1d0bfe9d4a9d657446c7d
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>