The qpluginloader machtest needs Q_AUTOTEST_EXPORT

Doing a build on OS X without -developer-build fails.

Change-Id: I49c178ab2428177d9dd94f84620595a4bc132244
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>

tests/auto/corelib/plugin/qpluginloader/qpluginloader.pro

@@ -5,7 +5,7 @@ SUBDIRS	=	lib \
                 theplugin \
 		tst
 !android: !win32: !mac: SUBDIRS += almostplugin
-macx-*: SUBDIRS += machtest
+macx-*: contains(QT_CONFIG, private_tests): SUBDIRS += machtest
 TARGET = tst_qpluginloader
 
 # no special install rule for subdir