load(qt_parts) causes the inclusion of src, tools etc
but on non-win32 platforms (and wince and winrt) we have
to add the src subdirectory anyways in order to create
the doc target.

Task-number: QTBUG-46712
Change-Id: I51cf4050fed744c844acc8ff4fe5f5f1d8c32d1a
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>