core as a directory name is usually not a good idea

core as a directory name is usually not a good idea

Let's follow the other places in qtbase where the
directory is named corelib.

Change-Id: Ib426f4ee7311f622a89b252b9915aca1d3dd688d
Reviewed-by: Casper van Donderen <casper.vandonderen@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>