Explicitly register gdb debugger for MinGW

Registering the debugger 'indirectly' through addKit breaks with the
new parameterized kit names. It's probably anyway saner to explicitly
register the debugger.

Task-number: QTCREATORBUG-13529
Change-Id: Ib6a5826efef9d9ff8b7c893a87d024fc324dfd9f
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>