Extract the definition of q20::ssize() from tst_qanystringview.cpp,
where it had to be placed for its backport to 6.2.
Change-Id: I3f758c98a4b1efd453f4fc044b8d3f1a89de62d1
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>