Reserve some space for future use in QLoggingCategory

Currently Qt offers only debug, warning, critical message types for
general use. Most logging frameworks offer more ... let's save
some space for future message types.

Change-Id: Icb4333da5c8f5277fd10d8a01b06d95369662bdc
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>