qstorageinfo_unix.cpp: Fix build on BSD and other unices

* Include statvfs.h on all non-Linux and non-Solaris systems.
* Fix type of stat_buf structure on BSD.

Change-Id: I6336503082fafd7f6108cf95c079bdd329d2ea0f
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>