Fix compilation of public headers with QT_NO_DEPRECATED defined

Fix compilation of public headers with QT_NO_DEPRECATED defined

Put the functions in QT_DEPRECATED_SINCE if possible

QPluginLoader::staticInstances is not documented as deprecated, and do
not reference any alternative use. So I unmarked it as deprecated.

Change-Id: I556c3f3657fb0490dd5543fcc56718fe9bd394e7
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@nokia.com>