Make prefetchData() virtual instead of pure virtual
Make prefetchData() virtual instead of pure virtual

prefetching is optional, so it makes more sense
for prefetchData() to be just virtual with an empty default
implementation.

Change-Id: Icd2303112d58de6c6c04f28d674979f8376d4120
Reviewed-by: Thomas Lowe <02ef35f2b5ffabc77f5e36c1ccbe561158ecef1a@nokia.com>
Reviewed-by: Alex Wilson <75fe28a89a52dd8f665f8aed10f0a66d41b4f1d0@nokia.com>

kobolabs/qtlocation,amccarthy/qtlocation,amccarthy/qtlocation,kobolabs/qtlocation,kobolabs/qtlocation,amccarthy/qtlocation