QWinEventNotifier: compile with -Wzero-as-null-pointer-constant

Seems to be the last 5.6 QtBase public header that didn't,
paving the way to add the warning to the headersclean check.

Change-Id: Ib2655782e34ec58e5d9b1b9c0ec31a965a38f9b7
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>