Fix agreement between func declaration and definition
Fix agreement between func declaration and definition

I added this bug in
    2020-04-06 DirInfoTask: use a single function for size and count
    37db563d4781e81b43330104e6ad0ec2506811b3

In a development version, I tried to have a `const QDir dir`.  That
didn't work, but I forgot to remove the `const` from the declaration
before committing the change.

Reported by:	clang-tidy

Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui