scripts/spdxcheck: Add count of missing files to stats output

Add a count of files missing an SPDX header to the stats
output.  This is useful detailed information for working
on SPDX header additions.

Signed-off-by: Tim Bird <tim.bird@sony.com>
Link: https://lore.kernel.org/r/SA3PR13MB6372DB9F9F2C09F8A1E1B99BFD1A2@SA3PR13MB6372.namprd13.prod.outlook.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>