Make public headers compile with -Wzero-as-null-pointer-constant...
Make public headers compile with -Wzero-as-null-pointer-constant
... or equivalent.
QtBase 5.6 headers already compile that way, so let the other
modules follow suit.
Change-Id: I79c74046eaf224235cc80a4323fefd2c083b007f
Task-number: QTBUG-45291
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@theqtcompany.com>